Our guild used to make scrappy log cabins for charity quilts.. We had one stack of lights and one stack of darks. You were not allowed to match up colors.. You would be amazed at how pretty they come out.. I think the most important thing is to make sure they are true darks.. not medium colors. If you are not sure get a red cellophane and look thru it, it will help you distinguish the darks and meds. I had a cousin that would try to match up colors,, I had to stand over her and make her take one dark and one light just randomly...when it was completed she would be amazed... Just expand your boundaries, by not being so restrictive.... Good luck