Color & Contrast
Relying on color and contrast to tell the details is something that could leave many of your visitors wondering what is actually going on. Consider a situation like the example below in which someone might used colors, instead of words, to describe the results of a race. The results are that of a foot race and as long as you are using a color monitor and you can see color fine it should be easy to figure out that Jon Smith received the gold medal, Billy Bob received the silver medal, and Jessica Smith received the bronze medal.
These are the results of the 7th annual run. The finishers are highlighted in the color of the medal they received.
It would be very hard for us to tell that what the list was meant if we were looking at this in black and white, our color perception was not well, or maybe even if the colors on our monitor were not right. Also if a user of the site were using a page reader it would not tell them that Jon Smith is a gold color so it would be impossible to figure out what the list of 3 names really meant. So the problem with this is that it is relying solely on the color of the text to explain the list and we want to make sure we do not do that.
These are the results of the 7th annual run.
Gold medal: Jon Smith
Silver medal: Billy Bob
Bronze medal: Jessica Smith
The above example would be a much better solution. We were even able to remove the sentence that said they were highlighted in the color of the medal they received because we say the color right next to their name. It is possible you could have left that sentence in there as well but it becomes repetitive at this point.
You also want to make sure that all images on your page show up fine if you print them out in black and white. That is the best test for all of this. Print your site out in black and white (gray scale) and make sure you can understand all your content still. It is a very simple process and should not take much more than a few minutes. Many people also find more spelling and grammar mistakes when they read on paper than when they do on a computer screen so you can use the printout to make sure the page looks right and to make sure your spelling and grammar are correct.