You have some great answers already, John. I suspect you'll have to ultimately decide what you feel works best for you.
I believe colored mats (usually double mats with two different colors) and a coordinating frame enhance my paintings. I've received suggestions to frame paintings for shows in simple, thin black metal frames with white mats for a uniform appearance for all my paintings. However, while that strategy makes sense in theory, it just doesn't help show off the colors and subjects well enough for my taste.
As for mat size, I believe there are "typical" mat widths for certain size frames. A fairly standard single mat that fits an 11x14" frame measures 2 3/4" wide on the sides and 1 3/4" wide on the top/bottom (landscape orientation). I'll reduce those by 3/8" for the outer (larger) mat when double matting, allowing that 3/8" to be an accent color in the painting as below:
I could have chosen many options for the mat & frame. Regardless, if that painting were in a white mat, I just don't think it would look as good.
Let us know what you find works for your paintings. Would be great to see some!