I use matching colors best/often as I can.but have not had any problem with one color on top and another color on bottom...as long as tension is good. But for generalities...I use a cream/beige/tan color for piece assembly and then matching colors for the top/FMQing. Unless I am using alot of dark colors...then I match closest possible for piece work. I hate having to have to many bobbins "ready to go". My new Singer Quantum...because you have to unthread the machine, rethread to wind bobbin and then rethread machine...I have to wind many....BUT on my good'ol Singer 2010 (now 28 years old!) the bobbin is so perfect....run out? open bobbin cage, press "one stitch" button, and it threads bobbin! I dont' have to unwind, unthread...nothing! Whendone winding the bobbin, i just close the bobbin case, pull the thread out the top and start sewing again! Boy did SInger make a HUGE mistake in design when they took away that feature!