Mesa EL-34 tubes colours?

[Tom_cheats]

hey whats with the yellow,grey and blue coding of the Mesa EL-34 tubes? does this mean anything to the sounds or combatability of them? when i go to buy them i find these three options and am confused

 

[primeholy]

This means they are matched and good to use with the same color coded tubes in the same amp. If you have to get tubes make sure the color matches on them all.

 

[Areola]

Here's a question that I got asked when buying new tubes... I had Yellow in my amp, but then got asked if I wanted to try Green ones because they're a little hotter. What does that do to the sound??