Let me state the rules for you:

1) There are no rules

That said, I usually match anything that doesn't clash with the colors of my kilt. Easy, eh? There are going to be people who feel the need to tell you "you're doing it wrong", this can't be avoided, but the only person who is qualified to police what you wear is you.

Be comfortable, that is job one, and wear your kilt with pride and dignity.