Ah, thanks, O!

Ambient color is in this case a light straight at the canvas, while the light color comes from a light you can place anywhere. What you can do, which will simulate a pure ambient, is add another layer and set it to additive. (That's the little button toward the center of a layer slide called "MODE". You can click on it to toggle the different modes layers have. You'd want Mode "+")
Then you can fill this layer in the ambient color you want. I'd pick a very dark version of it, though! Hit control + shift + F to fill the whole layer with that color.
After that you can control the amount of influence this layer has by its Visibility. (That's the last dial on the layer gui).
You will see that it has an influence on the whole picture, as such an ambient would have. That means you really have to pick your colors for light and ambient light and the fill for that layer carefully, if you wish for it to sum up to white! Tricky thing sometimes, but it's possible and a nice challenge, too, haha.

... I know.
But, yeah, that's one way of simulating that. And it is indeed how it is done anyway, except that you wouldn't know about it. Now you not only know about it, but you can even use this to play around in ways that otherwise would not be possible. You can paint on your "ambient layer", giving different areas slightly different colors. Of course, that would screw up the whole "sum up to white" aspect, but it may also look fantastic?!

... definitely worth a shot!

I'm really happy to have you here! Very inspiring!
