The pastel chalks (NOT oil pastels) are great for adding subtle color. The work best on a matte finish, they should be the last step in your finish work. Top coating will change their color, so don't top coat them. They are surprisingly durable, the pigment works it's way down into the matte finish and will not smudge when the model is handled.
They work great for exhaust, fading panels, etc. I scrape some pigment from the stick with a razor onto a pallete. You can mix the colors to get the shade you want. Apply them with a brush or Qtip, then blend with a paper blending stick or a small piece of toilet paper.