Not sure if you are talking about the contrast between the two? What color thread looks good with color of leather or just colors of thread preferred. For me, It just depends on what im working on. On some things that are plain or say just basket weaved like black police style holsters i like white thread that pops. On something like western holsters where carving and stamping patterns on the leather i want that to stand out so i would use thread that is the color or close to the leather color so the carving and stamping stand out. If you are doing a project that you are using two different dyes or coloring a carving you just have to use judgement. Also, it depends if you are hand stitching how good you are at it if you want it to stand out. I use Maine thread and the colors i like are Lark, GoldenBrown, Bark Tan, Black and White.
By the Way, the reason i got into leather work was because i wanted to eventually make cue cases. I have been doing it 3 years now and still havent attempted a case lol. Im going to attempt a small 1x2 probably this coming spring. That small 1x2 has probably cost me over 1000 dollars in last 3 years