Dark tones are brown during the warm up even after 30 minutes the colour looks abit too brown/yellow.

Please explain how can i fix this thing?

Either the CRT is defective or probably the capacitor in the heater supply circuit is low - I'd change this anyway as its a problem part. First of all, check to see if you get 6.3V on the CRT heater pins and if not, check the components in this supply - its fed via a transistor (which switches the heater off in standby mode) from the power supply. I don't have the schematic to hand at the moment but you should be able to trace it back from the CRT to the PSU.