Paint mixing

My first suggestion would be to try to lighten the two colors by adding a few drops at a time until you get the colors you want.

You can make a green useing a mixture of black and yellow - a 1/2 and 1/2 mixture will give you a very reasonable match to Zinc Chromate Green and by varying the ratio of the two colors make it darker or lighter.

As far as the brown goes, I need to check my color wheel to see what basic colors are used to make brown.