It's kind of difficult starting out, but you can come up with some general rules, and so long as you're consistant it'll usually work out pretty well. For instance, it's common to use brighter, warmer, or more saturated colours for foreground stuff than you would for background tiles. If you keep your palettes small it's relatively easy to keep track of what stands out against what. You don't have to switch out everything drastically - just change enough to make it obvious that there's a guy there.
not that you can't see him anyway - just that he should stand out.
I think red is a good choice, because it complements the green of the grass, and it's warm so it'll pull forward naturally. But any colour will work if you find the right shade, you just have to mess with it till it looks right.
