Sounds like "drybrushing". Typically, you'd use this technique in reverse, to brighten the edges of details with a lighter colour, but it could be used on "flat" surfaces to darken them, as well.
On this model, if you look under the model's left shoulder guard, you can see the blue wash that covered the entire model. I drybrushed the model up to white from that, which leaves some of the blue showing through. You could easily do that the other way around, and drybrush a lighter colour towards dark, leaving the light colour showing through. A "wash" would be ideal, as the pigment would be thinned, allowing more of the base colour to shine through.
More passes with the brush leaves more colour on the model.