Should Certain Colors Cost More?

Yesterday I went to RW & Co. and found a couple of dresses that actually fit me correctly. I think I have a weird body shape or something because it is very very difficult for me to fit a dress that I am comfortable in and actually looks decent. Well I talked myself out of buying them because well, I'm not sure why. They were both on sale and everything. Well lastnight I decided that today I would go back and get the little red one. So today I went back today and realized maybe red wasn't the best choice. So on the same rack there was the same dress just in charcoal and blue. So I tried the charcoal one on, I liked it so I decided to get it. I went up to the cash and it came to $54 something. I found this extremely strange since the Red on was only $29 plus tax. So once I paid I went out into the hall and checked the tag on the charcoal one and sure enough it was $49!!!! How can they charge $20 more dollars the same dress in a different color??? I think this is adsolutely craziness. But I still bought it and kept it, it was marked down from $88, looked good, and is perfect for the upcoming grad dinner/dance. Anyways, just curious do you think different colors of the SAME clothes should cost more/less?