Possible step one, overlaying two different light sources to make a new light that looks the same as some other light, but has a different underlying rainbow.

Mixing firelight and white-hot metal until they look the same color as sunlight, for example, adding more or less firelight to the mix, until it looks the same as the sunlight-source.  But trying to preserve the part where fire-light + metal+light should have a different rainbow than sunlight.

Then, practice shifting back and forth between 'rainbow 1' and 'rainbow 2' while keeping the appearance of the light source constant.

Possible step one, overlaying two different light sources to make a new light that looks the same as some other light, but has a different underlying rainbow.

Mixing firelight and white-hot metal until they look the same color as sunlight, for example, adding more or less firelight to the mix, shifting the light's tone redder or bluer, until it looks the same as the sunlight-source.  Possibly, if that's a thing you can even do.  But trying to preserve the part where fire-light + metal-light should have a different underlying rainbow than sunlight.

Then, practice shifting back and forth between 'rainbow 1' and 'rainbow 2' while keeping the appearance of the light source constant.