"I haven't quilted enough to be able to quess my thread needs yet. Any tips on how to do that so this won't happen to me again? "
go to the Superior Thread website and look for the education section. there is advice on how much thread different sizes of quilts take to quilt.
I hope the new color you are waiting for will be a good match but if it doesn't, I think something slightly lighter rather than darker would work better. Good Luck.