..find an exact complementary colour.
You night like to try this first with bright primary colours. For example find something red. A book cover, or napkin, or an apple, and a piece of plain white paper. Now gaze for a slow count of twenty, without blinking, at the red object. Then transfer your gaze to the white paper. You will see what is called an after-image which will be green, the complementary colour of red.
If you stare at blue the after-image will be orange, at yellow the after-image will be purple.
It works even with the most subtle colours. If you are trying to mix the compementary colour in paint, you have to memorize what you see on the white paper- takes practice - but you can always gaze at the original again to get a match.
You can probably look up charts, but this is more exciting! Let me know if it works for you..