Can I make something emit light that is a different color than it is?

Say if I had a cube that was glowing red and I wanted it to emit a blue light. How could I do this?

You can use the “Is Camera Ray” output of a Light Path node for the mix factor of a Mix RGB node and connect it to your emission color.

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This is a context dependant solution, you can also fake it by creating a fake blue cube with emission that is not visible to the camera itself, and another red cube with no diffuse visibility.

See how this solution looks like in Cycles

This was my solution to such an issue. I have the black and white texture painted runes on the wings connected to the color ramp with the color I want to emit plugged into the emission node input. I can then increase the intensity of the color with the slider. I hope this helps.

Oh, and I overlaid it on the original texture color input so it not only emitted the color but was part of the color input.

At some point in history, the computerized method of painting was introduced. Some brush based artists loved it, and others not so much.

That’s the way I feel about nodes. They just don’t make sense to me. Does anyone find this screen shot helpful? It must take forever to look at all those connections, to sort it all out.

Sorry, venting. [2.79 Internal ! - Forever]

Well in this case I am not surprised you are confused.
The original question was «If I have a red glowing cube can I make it emit blue light?»
The nodes for this are a lot more simple!

Edit: sorry to edit again!

Sorry for the confusion. In that screenshot I was simply attempting to show that the blue color ramp plugs into the the emission node on the principled shader to emit light of a different color than that of the wing itself. It is plugged into the bump and other nodes for different effects.