@AnimePewdz
I apologize in advance, but I love color theory and science! 😀
When talking about colors, there are a lot of colors in the human perception that don't even have corresponding wavelengths, like magenta, and opposites usually refer to the colors that mix together to neutralize each other.
If we're talking wavelengths, then there's no circle at all, just a single value representing length. On the short end, Planck length might create a hard limit, but as far as I know there's no hard limit on the long end. Some long shot hypotheses regarding dark matter involve particles with wavelengths that measure in light-years, which would (understandably) be difficult to directly measure, and there are some hypothetical mechanisms that could create wavelengths as long as the length of the entire universe. Although we have no way to measure how long that is, since we're limited to the visible universe. But in that case opposites would be Planck and Entire Universe.
You could also go with blackbody color temperatures and have black and blue be opposites. 😋