When I am sewing 2 high contrast fabrics together, I try to plan to sew them all the same way (light on top). Doesn't matter on top thread (usually use a neutral) but I use the dark color in the bobbin. This is because I press the seam to the dark side and if a high contrast thread, there is a slight possibility of it showing thru. With a good quality fabric, this probably isn't a problem, but it is a habit I formed years and years ago and it works for me.