If it is a complicated block, particularly if there is mirroring in the desing, I usually make a test block - also if I am iffy on the color. As for color, I usually channel my inner 5-year-old and draw the pattern on grid paper with color pencils. It's fun!
One thing I gotta say: My quilts NEVER turn out to be the size the pattern says and I'm OK with that. When people ask me how big it will be, I tell them that I will let them know when I am done. lol