Nice job for only your second kit. Don’t give up on 1/72 just yet. Like everything it just takes a little getting used to. There are several olive drabs, one for armor, one for aircraft, one for clothing, etc. You may have gotten the one for armor as it was darker than the one on a/c. Tamiya makes some really nice spray cans just for aircraft, and would have the proper OD. One of the plusses is that they are smooth enough not to need any Future before applying the decals. You should be able to get them through your local hobby shop. For masking the canopy, check to see if Eduard has a precut mask for your kit. Makes that job a whole lot easier. Welcome to the world of divine scale modelling.
Other than the couple of things already mentioned, I like this one! Well done, you should be proud your little Mustang. Those 1/72 birds can be a challenge!
Jeaton01 - Thank you for the additional info on using clear decal paper. Just curious, have you ever considered painting masking tape strips? I wonder if that would work.
Lewbud - Thank you for your comments on olive drab. I will take a closer look at the olive drab selection the next time I am at the hobby shop. Sadly though, the hobby shop most convenient to me became a victim of the recession and went out of business. Looks like a drive across town will required in the future. Of course, there’s always online ordering. There’s also ways to lighten the paint I already have. I’m thinking of trying one of those Eduard masks. I see one is available for a 1/48 P-40B although not specifically for the Revell-Monogram kit.