Very short version: if you don't think about what you're trying to convey with an illustration and choose colors appropriately, then you can make the important parts hard to notice and/or create illusions about what's going on. Yellow is brighter than red or blue, and this may lead to too much emphasis of the middle of the scale if you use rainbow mapping.
Careful use of a dark brown-red-yellow color range, logarithmic scales, 3-D maps, and gray-scale mapping can help.
Point not made in the article: I think that having all the images in the same rainbow colors makes it harder to focus on any of them.
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