There's no hotkey to set the foreground color, but you may be able to create an action to select a specific color. This will take up more space than it is worth to set various colors. I would recommend, if I understand the basis of your question correctly, that you take some time to build and create your own swatches and simply get used to flying over to the swatch palette or the foreground/background color boxes. You can even create a layer in photoshop documents with little squares of your colors, and just hit the alt key when you're in a brush or painting mode to grab the color.
definately do what vfxart said.... either set up your swatches or on a new layer put down a bunch of circles of colors you'll need in your painting. i do it with every painting and it makes selecting color so easy