Check your manual, but generally the power tubes are installed like this: X - Y - Y - X, where the X's are a matched pair and the Y's are another. The two pairs do not have to match each other, that's why Mesa doesn't sell them that way. You'd go Red - Yellow - Yellow - Red, or any other color, in that pattern. The colors serve only to match the tubes to each other, so any red works with any other red, etc. The various colors have nothing to do with sound quality.
There are some sellers that will match a quad for you, and get them to match Mesa's bias range.