There's no easy way, but it gets more instinctive the more you do it, you develop a feel for it. Cut a whole out of some white paper, and place it over the colour you want to match (go for the lighter colours first if possible, then you can use that mixture to gradually darken if needed for other tones) mix up a colour for example light green, yellow, bit of blue, white then spray onto the paper near the hole, see how close it is and adjust form there. I think some people do mix up all their colours first.
I am too lazy so do it on the fly. Also as I use transparents, colours emerge on the surface as I paint sometimes, for example blending yellow over red for oranges etc. Plus I'm not always that bothered if its a 100% match depending on what look I'm going for, or what paints I have. As long as the painting looks right as a whole. But then I'm no photorealist and rarely copy any image exactly, putting my own slant on things if I can.