Kp51ws500 color flickering

I'm afraid I don't have good news. This is known as the "Flicker Issue." As a Sony authorized servicer, the shop I worked for was directed to replace all three of the CRT's in the limited number of sets that experienced this issue. The cost per tube is typically around $200. If you decide to take this on, it is best to replace one CRT at a time, power up the set and do the yoke adjustments and electrical alignments to get the new color to line up with the existing ones.