Thank you. FYI, this is for my XPS 420. Am I correct in believing the bulbs yellow color does not mean the PSU is AOk i.e. putting out correct wattage?
That is correct.
The LED on the motherboard is the 'Stand By Power Light' [aka 'flea power'], indicates the motherboard has +5VFP power is reaching it and not an indication that the power supply has not failed, or has no issues.