"What's the color of the shirt that you are buying?" Is a bit better . And color is just fine, there is not need to change it to colour. The difference is that in British and Canadian English we write colour with a 'u' and in US its written color without a 'u'. There is really no difference.

"What colour shirt do you want to buy?" To follow up on one of the entries below: color=American colour=British

I think that you could say: What's the color of the shirt that you want to buy? or Which color of the shirt do you want to buy?

Yes just the word color must be "colour"