Well even a yellow DO is better than a green training / general. So I say upgrade as soon as possible, but not at the expense of your other powers. If you manage to just afford one DO for one power, and so all your other powers are full of yellow training enhancements, maybe not quite worth it. I'd say try to keep the majority of your enhancements white, if not green. THEN go for a DO (I buy a new DO everytime I've saved up about 6k of influence).
Coal's main problem is endurance, so my first like 5 DOs were just endurance reductions. Then I went with accuracy for my two main attacks. Then I replaced both of my damage enhancements with DOs in freezing rain, just cuz I like it - prolly woulda been better served putting 'em in my main attacks, but hey sometimes ya just gotta do shit for fun eh? I've got about 9 now I'd say, anyway that's my take.
On that note, anyone know the exact percentage difference of different colored enhancements (like green vs. yellow vs. white) - or is the number all that matters and the color (with the exception of red, which means it's shut off completely) just to make things easier? I'd know, but my strat guide has mysteriously disappeared.... >

Isn't it like a 1% difference per level? So if you're level 8, and you have a level ten enhancement, it's 2% more effective than if it was a level 8 enhancement?