Mesa/Boogie power tubes are color coded according to being matched in testing. This is dissimilar to some other manufacturers where the color code indicates different levels of gain/output. The Mesa tubes are color coded simply to keep them in matched pairs or quartets.

You CAN mix two color codes in a 4 power tube setup, but one color needs to be the outer most tubes, and the other color needs to be the innermost tubes (push/pull). This doesn't mean you will "blow up" the amp if you install two colors incorrectly.

As an example, a 100W amp could use RED in socket 1 & 4, while using BLUE in socket 2&3.