I have been to all sorts and conditions of weddings, but my experience is the kind that gets planned months in advance tends to be a fairly well-thought out (i.e., rigidly organized) affair. I have read advice above that suggests that a kilted groomsman is not expected to look like the other groomsmen at all. I have not noticed that advice coming from anyone who might be called "the bride", who I believe is the person making the real decisions.

If she is deciding in advance how the other groomsmen will look, she is probably not interested in hearing about how much the groom's brother likes this color for ties or this color for shirts. She has chosen and she hopes he will wear it, or else he can sit in the back and sing with everybody else.

Here is a simple clue. Many weddings now include a program of some kind. Home computer typesetting and printing make this easy to do, usually with a ribbon attached, but of course, there are printers who manage this along with the invitations...

Anyway, imagine the program, if you will.
Does it say
Groomsmen/Ushers
Mr Rowan Atkinson
Mr. Hugh Laurie
Mr.Stephen Frye
Mr. Tetley 88 Mark

Or does it say

Groomsmen/Ushers
Mr Rowan Atkinson
Mr Hugh Laurie
Mr Stephen Frye
Groom's kilted Brother
Mr Tetley 88 Mark

If you are counting on the latter, then, by all means, substitute whatever color of shirt you please, whatever style of tie you like, whatever else, because, as the kilted guy, you are special and entitled to do what suits you. But if you expect the former, then you had best start thinking about how the bride wants you to look.

Or maybe it is just me and the rest of you gents have dealt only with brides who do not have a preference for anything.