I know last week I spoke briefly about contrast from a visual sense and how that effects an image, and draws a viewer in. In the course of doing that, I "discovered" there are different TYPES of color blindess. Under these types, there are underlying catagories "based on the number of primary hues needed to match a given sample in the visible spectrum."  So depending on the color blindness and the subcatagory of it, it will completely affect how an image will appear. The sub catagories are monochromacy, dichromacy, and anomlous trichromacy. I'm not going to go into the details on each, but rather the visual effects some of the subcatagories have on visuals - but if you're interested in more details, Google it.
This kind of blew my mind, because all that I've ever heard of is red-green and blue-yellow. So if you take a rainbow and how a normal person would see it, it would look like this:
If a person has no perception of red what so ever, which would be a type of dichromacy, and red would appear dark, almost like a dark brown:
If a person has no perception of green, it will also affect the ability to tell the difference in the red and green hues, which mean the yellow and blue pigments are the primary colors seen:
AND THEN if there is a total absence of blue pigment receptors the rainbow will look like this:
Fascinating stuff, isn't it? A bit mind boggling at the same time. But why is this important to consider, as a designer, as a developer? Because we put stuff on the web to be consumed, and we want it to be consumed by everyone and be accessible to everyone, and we can do that with contrast, "the percieved difference in colors that are in close proximity to each other" !
There are different ways to approach this. Some designers will start a design in black, white, and shades of gray. Others would use tools like graybit.com  or Color Scheme Designer 3  to make sure the color scheme would work. The latter sucked me in for a bit, as I played with it - good times.
I also tested some of our current feature graphics with Fujitsu ColorDoctor  to see how we've done with our contrast in the grayscale conversion, and I've noticed that we need to up the contrast on the background color, for some of them.
We've got a bit of work to do, but learning new (to me) stuff is fascinating!
*Image samples of the rainbows are from the wikipedia.com entry on color-blindness