Well, there are certain colors that work with pretty much any tartan, and then there are even more colors that work with a large number of tartans.

For example, having hose in varying shades of greens or blues will do you well with many, many, tartans, given the predominance of these colors in most tartans. Even tartans which are mostly red often have minor blue or green elements which the hose can pick up on.

Many colors are more neutral, such as black, charcoal, grey, tan (khaki, oatmeal, etc), and you'll find these tone well with just about anything.

If you are looking for hose that will work for both your MacLeod of Harris and MacLeod of Lewis kilts, you can start with the colors they have in common, which are yellow, red, and black. I'd say red hose would work well for both kilts. Black would, as well, and likewise charcoal (which as a dark grey would tone well with the black in the tartans). Yellow would certainly look good with the Harris kilt, but I think that it might be a bit "too much" paired with the Lewis kilt.

I'm not saying that ONLY colors that appear in the tartan will work, but they do give you a good place to start.

And please note that it is not necessary that the color of the hose be an exact match with the tartan, only that they tone well together.