The stand-alone Atari800 emulator, not the Retroarch lr-atari800, you can goto the menu (F1), then "Display Settings", then scroll down to "Color preset" and select "Vibrant". That fixed the washed-out look, for me. There is also a manual "Saturation" setting, under the color preset, so you can further increase the saturation, if required. Mine is set to 0.28.
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