With colours and lighting, it's just a whole lot of experiments. For this one, I first made a smaller, rougher sketch in grey, yellow/orange and blue/green to come up with a basic composition. I fixed where the lighting is meant to go with an overlay layer with blues and yellows.
When I painted it properly though, I used my sketch as a reference and bit by bit put in colours on a de-saturated green form. First the yellows for the light, then the oranges and reds and the dark desaturated yellows for the in betweens, and then the blues and purples for the 'shadown'. It was trial and error, that's the way I usually go about it.
As for whether or not the values are correct, I'll occasionally do this thing where I'll add in a layer set on blending mode 'colour' and fill it in white. If this layer is visible, the whole piece will look black and white. Why I don't rely on it mostly is because hue and saturation also effects our perception of values, which the softwares don't always consider. At least medibang definitely doesn't. So the B/W version will look like it has different values to the colourful image.
TL;DR: I use a lot of trial and error.
I don't know if this would actually be helpful to you, but I'm flattered to be asked and I hope I was some help. ^^ Sorry it got a little long. ^^;
Since I'm painting skin and since I like these colours anyway, I went with reds and (since there's both red and yellow from the warm light source) orange. The rest were a result of ublending and using the eyedropper tool. If you hold your hand up to a light, it'll glow red, that's why I chose red and oranges.
I actually have the progress shots here: https://www.artstation.com/artwork/bKmN8G
No problem at all! ^o^/