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I have some data I want to represent on a choropleth. It's a divergent data vector, but the data has an additional dimension that is a 2-value factor. Some states are "yes", some are "no". I'd like to simultaneously use two diverging color schemes for the data vector, one scheme for "yes" states and one for "no". The trick is to have all four extremes (yes/min, yes/max, no/min, no/max) be immediately distinct, particularly a four-way contrast that would be visible to the color blind. What choices would be optimal for my four "extreme" colors?

I think I need to explain what "diverging" means. A diverging color scheme is one with a "center". Values can be lower or higher than the center, but what is meaningful is specifically the relationship to the center. Thus, "low to high" is the wrong sort of color scheme to use.

Note: accessibility for color-blindness is preferred.

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    Any sample data plots for us to view? A picture is worth your 82 words. – Stan Aug 25 '17 at 16:04
  • I agree that a screen capture is going to be really helpful. It does not have to be the diagram itself, it could be just a sketch. – Rafael Aug 29 '17 at 11:08
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If I understand you correctly, it seems to me that what you need is ColorBrewer – an excellent tool for picking colour schemes for cartography.

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  • You just made MY life simpler. +1 – Stan Aug 25 '17 at 17:54
  • i aim to please ;) – benteh Aug 25 '17 at 17:56
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    I have used ColorBrewer. I am afraid ColorBrewer won't do it. It can handle one-dimensional choropleths well enough, but I want something that will have "four-point" visual contrast at the extremes that will likewise be able to be perceived by those with the more common forms of color blindness. – Bryan Aug 28 '17 at 12:05
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In a map you want to highlight two states of being, each of which contain two states of being.

Size is already a factor in your choropleth, so you have only color and pattern to differentiate them.

You could use light and dark green for one group and light and dark red for the other.

You could use four different colors for the 4 states.

You could use 1 color for each of the first group and then indicate Yes or No with patterns.

You could use one color for each group and have a N or Y placed on each.

Would you like more suggestions?

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  • It is not 4 states, it is 50 states. Likewise light and dark green vs light and dark red are not diverging. In addition, I would never contrast red vs. green ever, under any circumstances. Red/green colorblindness is the most common form of colorblindness. Red/green is the worst possible color combination. I will have to see if there is a way to implement patterns on a google geochart. – Bryan Aug 25 '17 at 16:58
  • Sates of being, my friend. Please use the color combos you find to be the most divergent, pleasing and contrasty. – Webster Aug 25 '17 at 17:01
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    I was hoping for something based on actual research of color perception, particularly contrasts that would be visible to the color blind. I like to adhere to that principle in my designs – Bryan Aug 25 '17 at 17:45
  • @Bryan Add this requirement to the question. There are many sources to verify the effects of various visual handicaps at allaboutvision.com – Stan Aug 25 '17 at 18:19
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Pick any two hues from the "Colour Wheel" that turn you on. One hue for YES and its complement for NO. For one extreme of each use a tint and for the other use a shade. Use these four vectors to fill the outline of each of the 50 states in your illustration.

Here is a web safe pure hue sample that is appropriate for normal vision using two hues and their complement. You can use any normal colour picker in PhotoShop to get the hues in the notation that you prefer.

Web safe two-dimensional divergent colour scheme for normal vision

Less attractive are the Web safe hue samples appropriate for a colour-blind reader with protanomaly, protanopy, deuteranopy, deuteranomaly, trianopy, trianomaly, but not achromatopsy or achromatomaly. The RGB coordinates that I chose are noted above the samples.

Web safe two-dimensional divergent colour scheme extremes for colour deficient vision

Assign any values to the continuum that you like or choose your own for the (evidently) secret values you wish to plot.

Have fun. Good luck. Thanks for sharing.

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  • In this example, I used value as a vector but texture could also be used with hue. You may have one extreme shown textured and the other extreme without texture. The variations are endless. – Stan Aug 25 '17 at 17:42
  • How is that a diverging scheme? What you describe are continuous schemes, not diverging. – Bryan Aug 25 '17 at 17:46
  • Stan. This is probably not the best solution because of the color-blindness requirement. Red and green are complementary in some color wheels and the main problem in it. – Rafael Aug 28 '17 at 12:46
  • @Rafael The color-blindness requirement does not preclude complements. My color selection link is in comments to the OP. The OP refuses to cooperate by supplying any more information. I think the question should be deleted as being too broad. – Stan Aug 28 '17 at 14:31

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