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Old 06-03-2019, 08:54 AM
 
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Why do most of the graphic representations of rainbows that I see on bumper stickers, window decals, and other places, have only 6 color bands?

Haven't the artists heard of Roy G. Biv?

Don't they know about Duke Ellington's cow?
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Old 06-03-2019, 09:18 AM
 
Location: Aurora Denveralis
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The graphics field ran out of indigo ink years ago.

If you've ever looked, the region between blue and violet is pretty indistinct. You have to have a cartoon, Disney, or leprechaun grade rainbow to see it.
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Old 06-03-2019, 03:14 PM
 
Location: Somewhere in northern Alabama
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Why do people ask rhetorical questions?
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Old 06-10-2019, 10:27 AM
 
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Quote:
Originally Posted by harry chickpea View Post
Why do people ask rhetorical questions?
It's not rhetorical.

While not exactly heart-attack serious (I know, I've had two), I'd genuinely like to know why most graphic rainbows that I see have only six color bands.

Indeed, all of the divisions of the colors of an actual rainbow are rather indistinct, and I suppose one could divide it into six bands, but seven is the convention. At least it was in the physics classes I took (Roy G. Biv). I'm guessing that the more art-oriented folks who produce rainbow graphics just didn't take those classes. Or maybe the Duke's cow really did drink the entire supply of that color of ink.
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Old 06-10-2019, 11:16 AM
 
Location: Aurora Denveralis
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Why aren't all illustrations of birds detailed down to the last feather?
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Old 06-10-2019, 12:35 PM
 
Location: Somewhere in northern Alabama
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Quote:
Originally Posted by P47P47 View Post
It's not rhetorical.

While not exactly heart-attack serious (I know, I've had two), I'd genuinely like to know why most graphic rainbows that I see have only six color bands.

Indeed, all of the divisions of the colors of an actual rainbow are rather indistinct, and I suppose one could divide it into six bands, but seven is the convention. At least it was in the physics classes I took (Roy G. Biv). I'm guessing that the more art-oriented folks who produce rainbow graphics just didn't take those classes. Or maybe the Duke's cow really did drink the entire supply of that color of ink.
In printing, the two primary processes for web press are four color process and spot color. IIRC, the presses where my stuff was printed had the capability of six inks (heads), but the perf bars limited that. CMYK has a limited color gamut, especially at the extremes, where using the base Magenta ink results in reds and violets looking similar and indigo being problematic.

When converting pure colors, such as in a rainbow, the black ink would create a shade, and is avoided, so the rainbow gets made from mixing the three remaining inks. The result has to have similar reflectivity or luminance to look right on the printed page. In an ink jet or process printed photograph, a rainbow doesn't look exactly "right",(light doesn't come from it, and there may be some black used in the mix) but our brain accepts it as a convention.

Convert a rainbow into bands and ink issues become more troublesome. Yellow is high luminance, cyan less so, whereas pure magenta can appear dark. Colors then get chosen from a palette of colors that can be standard mixes of inks that go well together. Six bands seem to work. Seven probably don't combine as well. Also, if spot color is used, six heads or stations may be all that a t-shirt or flag maker has available.

Part of what might also be confusing you is that color names are not fixed, nor are they consistent. You can easily research that red was once inclusive of orange. Robin red breasts are actually orange in current color names, but a few hundred years ago were known as red. Orange was later named after the fruit (not the other way around). If memory serves me right, there is some tribe in central/south America that can distinguish at a glance many more variations of blue than people in the U.S. In other words, the division of the spectrum into colors represented by words is arbitrary, and thus the rainbow, which is a spectrum of colors, also has arbitrary names and numbers of colors.

If you want to get a sense of the range of colors we can economically print, find a place where you can buy a Pantone chip chart, used for color mixes for spot printing. I had to resort to specifying those to get proper spot color printing. (FWIW, Pantone also has a system of colors using six inks that is superior to CMYK, and some of the high end theft resistant concert tickets are made on presses with as many as 24 heads, allowing varnish, metallics, holographic, luminescent and other effects.

Lastly, once a convention is established, others follow. "Gee, that pride flag only has six colors but looks good, copy it for me." Similarly and as example, the emoticon here for a smiley face is still just an upgraded : and ) . A rainbow might be shown better in the RGB of a screen (which is STILL not actually correct), but a six banded one will be dutifully copied and used.
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Old 06-10-2019, 12:47 PM
 
Location: Aurora Denveralis
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Quote:
Originally Posted by harry chickpea View Post
Lastly, once a convention is established, others follow.
Really the only relevant sentence in that dissertation.

Four-color printing limitations probably have only collateral influence on centuries of drawing rainbows with a limited and discrete number of color bands. (I've snorfed up a lot of print shop fumes in my years. )
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Old 06-10-2019, 01:14 PM
 
Location: Somewhere in northern Alabama
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Quote:
Originally Posted by Quietude View Post
Really the only relevant sentence in that dissertation.

Four-color printing limitations probably have only collateral influence on centuries of drawing rainbows with a limited and discrete number of color bands. (I've snorfed up a lot of print shop fumes in my years. )
When you say "Really the only relevant sentence in that dissertation." that really only applies to you and your singular opinion, doesn't it? One liner snark doesn't add to the dialog, nor does it reflect well on you, nor is it wise to get into a p-ssing match or attempt to troll with such nonsense.
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Old 06-10-2019, 01:16 PM
 
Location: Aurora Denveralis
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Quote:
Originally Posted by harry chickpea View Post
When you say "Really the only relevant sentence in that dissertation." that really only applies to you and your singular opinion, doesn't it?
Not really.

I apologize if that came over as snark; your post was informative and accurate as to the technological end of things. I just happen to think relatively modern printing limitations don't have much to do with why such a complex and subtle natural phenomenon is rendered in simplified form. It is, as you noted in that one sentence, a long-standing (many centuries) convention that is accepted by pretty much everyone. Further... explication is speculative and probably doesn't add much to an understanding.

But we can get into the expanded gamut of hexachrome and CMYK+Spot printing if you think it would help.
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Old 06-10-2019, 02:13 PM
 
Location: Somewhere in northern Alabama
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I understand that if you are thinking of high-end printing being the norm, limitations are fewer. What I am considering is that a LOT of grass roots movements start out with a single silk screen home setup, and that a next step for many is a six color t-shirt press. A search for t-shirt silk screen press turned up this in a flash:

https://www.lawsonsp.com/technical-s...nting-business

Six heads, inexpensive, lots of cheap product - Boom. Stuff fits together nicely.

I have a distant cousin who is into the t-shirt and tchotchke thing, so was curious what could be done, came across that 6 head press idea as fairly common.

Of course we could get into the nuances of screen printing as well,, if that would help...
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