I just fixed my SNES with this problem, monochrome output on composite and S-video. Occasionally some color would appear. It was due to the master clock crystal frequency drifting slightly with age. To fix, use gamebit driver to open SNES case, remove just screws holding metal shield over circuit board (leave power switch in place, as you don't want it shorting with board), then reconnect SNES to TV and put game in, power on, observe monochrome output, then slightly adjust the small screw in the red component near the front. Mine had to be turned slightly counter-clockwise, but try both directions. Find point where color just fixes, then turn about 20 degrees farther so that it's not just about to go monochrome again. This fixed the color on mine and now it works fine.
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I just fixed my SNES with this problem, monochrome output on composite and S-video. Occasionally some color would appear. It was due to the master clock crystal frequency drifting slightly with age. To fix, use gamebit driver to open SNES case, remove just screws holding metal shield over circuit board (leave power switch in place, as you don't want it shorting with board), then reconnect SNES to TV and put game in, power on, observe monochrome output, then slightly adjust the small screw in the red component near the front. Mine had to be turned slightly counter-clockwise, but try both directions. Once you find a position that works, turn it more to find where it stops working, then turn it back so that it's not right at the edge of working. This fixed the color on mine and now it works fine.
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Also see the answer below about replacing the crystal in case just turning the screw isn't enough.
I just fixed my SNES with this problem, monochrome output on composite and S-video. Occasionally some color would appear. It was due to the master clock crystal frequency drifting slightly with age. To fix, use gamebit driver to open SNES case, remove just screws holding metal shield over circuit board (leave power switch in place, as you don't want it shorting with board), then reconnect SNES to TV and put game in, power on, observe monochrome output, then slightly adjust the small screw in the red component near the front. Mine had to be turned slightly counter-clockwise, but try both directions. Find point where color just fixes, then turn about 20 degrees farther so that it's not just about to go monochrome again. This fixed the color on mine and now it works fine.