I have a regular photo and I'd like to try to give the subject eye make up similar to this:

http://i13.ebayimg.com/05/i/000/da/af/499e_1.JPG

How difficult is it? I've tried adding a color layer and painting the red on but it comes out looking very unnatural.

Try that using a curve ajustment layer.

I'd be inclined to use a light tone of the color needed and paint into a new layer with the Multiply blend mode. JMB's approach with a Curves adjustment can also work well, as could a Hue/Saturation layer (in normal or Multiply mode). I'm sure there's others...

Do a Goole search for "digital makeup tutorial." There's a lot of very good information available.

http://www.google.com/search?hl=en&newwindow=1&sa=X&oi=spell&resnum=0&ct=result&cd=1&q=digital+makeup+tutorial&spell=1

I'm not an expert in this (more in that I'm not MUA), but I've had great results using Hue / Sat ('Colorize') adjustment layers in Color blend mode for entirely new colors or Selective Color in Normal blend mode for slight adjustments. Just depends what is needed where you're working.

I have a regular photo and I'd like to try to give the subject eye make up similar to this:

http://i13.ebayimg.com/05/i/000/da/af/499e_1.JPG

How difficult is it? I've tried adding a color layer and painting the red on but it comes out looking very unnatural.

I hope you don't mean to say she's looking very natural now, do you?

If you post a hi-res image file we can experiment with it and show you.