Mixing the paint to match the colors is just trial and error. Get it as close as you can knowing that the paint will darken ever so slightly as it dries. When I think I got it close I just dab a little on the same color on the playfield to compare it. If it's off just wipe it off with a wet rag, and repeat.

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Which brush I use depends on how big the spot is I'm touching up.

Like Wawhite said above you definitely want to lay clear over your touchups to preserve, and protect your work.
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-Paul