Lots of good advice above. I do what the others have said. Also, if my card has a stamped image that needs to be colored, I stamp several on my white cardstock. Then I can try different colors out on the image to see what looks best with the colored cardstock I'm using.
Not gluing anything down until you're happy with it is very good advice. I can't tell you how many times I've changed my mind about making a card layout horizontally, 5-1/2" across, instead of vertically, 4-1/4".
I always make cardfronts and adhere them to my white cardstock base, so if I do change my mind, I haven't used a whole sheet of colored cardstock, just a piece 4-1/4" x 5-1/2".
Bugga in OK
"Be kind whenever possible. It is always possible." Dalai Lama