Is the black & white picture clean and clear? or is it also snowy and black/white?
if it's snowy then you may need to adjust the frequency adjustment in the RF modulator section. over time parts change value and may have
drifted off frequency slightly.
if it's clean then i would check the cap next to the cartridge port as suggested, I have also had this issue with the very first colecovision I owned in
the 80's.