I throw them in a pile and grab whatever comes out?
Honestly you just have to look at the fabric side by side with an idea of how you want to use it in your pattern. Remember that black tends to become a background and brights/lights will look closer to the eye. Other than that... do what seems right to you. I once made a quilt with orange, blue and purple. I didn't think they went together. But it turned out really nicely. So feel free to take a chance on any color choices.