All of the above.

What happens is this: When you first turn on the receiver, assuming that you follow the TX first and RX second (what we are usually taught from the beginning as the process to turn on the electrical power to a model controls) is that if the control surfaces are not exactly centered, the servo's will self correct or center to where they belong. That will cause an initial flash of green, followed by some "movement" of the lights toward the yellow, then it should go right back to solid green (fully charged RX battery).

Now, if a control surface is stuck, jammed somehow, that servo will try to center and draw current as it does so. And probably chatter. This will move the green light and may to to yellow until the problem is cleared. This means that it is drawing a lot of current to correct the servo position. If this happens, shut down and see if you can identify what's wrong and fix it before flying.

If it goes to the green right away as it should, then wiggle all sticks, usually to full deflection and watch the voltwatch. The lights will move according to the load you put on the battery caused by servo movement, then when movement stops, should go back to the first green LED (not the one next to the yellow, but the one on the opposite end) indicating you are good to go.

If it moves into the yellow when moving, identify what servo is loading things down a bit and see if it's ok. This could be a load caused by all servos moving at the same time as you wiggle both sticks, or if you have one that is binding, (throttle usually), then it needs to be adjusted. This is usually caused by the throttle linkage being forced to move "beyond full or idle" which will push against or pull on the linkage as the servo reaches the end points. Adjust this so that full throttle stick movement moves the throttle only to full and not attempt to go beyond.. and back to idle only to the limits of movement of the throttle arm and not to overload the servo by trying to move the throttle beyond idle. It should be smooth and not end up limiting on the throttle on the carb.

With all that said, the same goes for all other controls, but is usually the throttle at fault. Unless you are trying to push the control surfaces beyond what the hinges and hinged points will allow.. This should all have been set before you even maiden flew the plane, on the ground, on the build table so that you don't have these problems at the field.

If all is adjusted right, the indication (not a calibration by any means, by the way) should give you a green with small "excursions" toward the yellow. You may have a flashing yellow when moving the sticks, but never red. Depending on the load, this is normal. When flying, you will probably never move all the sticks all the way at one time.

The voltwatch is not an ESV and should not be used in place of a good ESV, but as a spot check of the battery condition and as a tool to monitor servo loads on the receiver battery pack before and after flight. It will give you a "heads-up" on battery charge level so that you will know when it's time to charge or if you have enough to continue flying. No more, no less.
CGr.