It's not a rule, but in general, if your wearing blues or grays your leather accessories look better if they are black.
If your wearing browns or greens the leather looks better if its brown.
With reds you can go either way, but black leather usually is more dressy than brown.
The only other thing which you should be aware of is to have all the leather match. Brown belt with brown shoes, and black with black.
Because tartan kilts can be all colors (sometimes all at once) us your shirt color as the base. White shirt, black shoes.
I hope this helps.