It seems obvious to just mix brown into yellow to make yellowish brown or golden brown, but if you do it this way, the color can easily end up looking muddy. I found this out for the first time when I did this painting.
To fix this issue, I mixed a tiny bit of red, an almost imperceptible amount in fact, into my yellowish brown mixture. This brightened the color, without making it look ruddy.
I actually got the idea to try this from the makeup artist, Bobbi Brown, who describes using red lipstick to brighten other colors in her book "Teenage Beauty".
In addition to using this little color mixing trick in the painting above, I also used it for this golden retriever.
This will not work to brighten green, though as red and green are complements. If you want a bright green, the only way I can tell you to get it is to buy one in a tube.
Painter of portraits and wildlife