Kinloch Anderson Kiltmakers and Highland dress in Edinburgh, and they said all the leather should match, shoes, sporran, sporran belt, kilt belt, kilt straps, the lot
In theory all matching leather sounds great, but aren't about 99.9% of the kilt straps out there black? Personally I stick with black simply because all of my kilts look better with black shoes/boots and my only 2 sporrans are black. Well, black leather on the day sporran and black and grey seal for the dress. I have yet to see a brown kilt strap in real life, so I just kind of think of the straps as a "neutral". My inclination otherwise would be to put a brown sporran with brown belt(s) and probably shoes too. I do the same for cowboy boots; belt and boots must be in the same color family for any self respecting cowboy.

My next two kilting projects are both blue based tartans, Ramsay blue modern and an unnamed blue tartan from Frasier and Kirkbright. I happened to find a dark navy leather scrap at Tandy, and since it look great on the tartan, will use that for the straps. But I have absolutely no inclination to try to find a navy blue sporran or shoes!