(Sorry if this is yours. The google found it.)
An alternate to this version of highlighting is wet blending. Wet blending, as the name implies, is blending together colors on the mini before they dry. The easiest way to wet blend when you're painting is with oils. If you're painting an army up for a tourney, client, or other deadline, you may not have time for the extended drying time for these guys to dry (some can take days). Acrylics, while not as good/easy as oils for wet blending, provide another avenue for mini painters to get a good blend.
I have been playing around with different solutions for wet blending with acrylics and the following seems to work best: 10/1 Distilled water/Winsor & Newton Flow improver.
Distilled water is super cheap and you can get it by the gallon at your grocery store. You want to use this over tap-water as it lacks all of the extra minerals and contaminants that most tap water contains (this can mess with your paint).
Winsor & Newton Flow improver can be found at any craft store or the wonderful series of tubes we call the interwebs.
When you use this mixture, generally it will be at a 1:1 ratio. This will vary depending on the level of pigmentation of the paint brand you are using (I did my tests with Vallejo Model Colors/Game Colors). A good way to test that you have the right consistency is when you put some on your brush and paint a small line on your palette, it will be transparent or "bead up" on itself. If you're too thin that's ok. Generally the thinner you go the better (you'll just have to apply more coats).
The cape on the second figure from the left was done with this ratio. First, base the area with a dark coat and allow it to dry. After the base has dried, apply a brighter color mixed with the dilution ratio to the raised areas of the cape. Rinse off your brush and dry it off. The paint on the raised areas should still be wet (don't do too many at once or they will dry). Blend in the wet paint with the surrounding area. You will have to do this several times as the underlying coat will show through due to the highly diluted paint. Mix together the base coat with the dilution ratio and come up from the recesses to blend in with the raised areas. This will obscure the lines between the colors you have made. Again, as you are working with acrylics, it is good to only do a small portion of this at a time so they don't dry. Repeat this process until you have reached the desired level of highlight. You can also add brighter or darker reds to make a more pronounced contrast.