Contrast - my tip to to look at the value of the fabric,. Is it a light or a med? does the fab fall in the middle of these two? If you put them together and stand back a ways can you clearly see two diff colors or do they blend? I prefer to use a really light / med/ and a dark. Think of the values on a scale of one to ten. then place your fabric on that scale. A white as a 1 and a black as a 10. A dark green would be a 10 a white with a green print would be a one.
Sometimes it is hard to decide the value level. I break them down into light-med light- dark light / llight med- med- dark med / light dark-med dark - dark
Hey maybe this should be a lesson to post LOL