One way to troubleshoot those transistors, failing spare parts, is to simply swap two of them. Swap the blue with, say, green. Now, if you have no green, but now have blue, then the transistor is at fault.
Similarly, if you have a stuck color (haze all over the picture in one color), you can remove the associated drive transistor. If you still have that colored haze after that, then the tube has a shorted gun. If the colored haze goes away, then the transistor was probably shorted (or there was something else wrong further back in the circuit).
-Ian