Hi, I'm new to 40Konline, but not to miniature gaming/painting. Good on you for putting brush to mini!
Since you're not painting *lots* of faces, you can probably afford to take a bit of time getting them right!
Before answering "How to paint Mexican skin-tones?", I'd first think about how you paint "regular" skin tones. Personally, I use 3-4 layers/shades when painting faces. Don't be afraid to try mixing your paints a bit - you'll need to thin the paint anyway for doing faces, so it isn't much of an extension to mix in an extra color.
I start with a mid-tone, then wash liberally with a dark gray-brown. Most GW browns are too red for this - I use a 1:1 mix of Scorched Brown and black. Make sure the wash is thin enough that it doesn't completely black out the face - you want it to dry so that the raised features are just a little darker than before you applied the wash.
After the wash is dry, I go back and reapply the original mid-tone, painting from the top down and trying to leave the shadowed surfaces dark. Next, I use a highlight shade (add a bit of elf flesh/light brown/tan to the base skin color) and drybrush that onto the face, again applying brushstrokes from the top of the head down towards the body. Try to avoid creating any strong dividing lines at this step.
Finally, I apply a light glaze (same as a wash, except you don't drown the whole area, just paint very well-thinned paint over the bits you want to add color to) to the face, using a shade just a little darker than the base color. This takes the shine off the highlighting and ties the face color together.
For Mexican/Hispanic skin tones, I would consider using Bronzed Flesh mixed with either Dark Flesh or Snakebite Leather as the base color. Since you're new to the hobby, you may be limited by your existing paint selection. In that case, start with whatever basic flesh color you have, and mix in a *little* brown until you think the tone looks right.
Best advice is to try something and see how it turns out. Paint the face first, so you aren't investing time in other parts of the model. If you don't like how your first attempt turns out, strip the paint off with Simple Green and have another go!