I always had the same problem because of the transparency, even when a color is labeled as opaque. What I ended up doing was painting highlights with small highlight strokes of titanium white and then going over it with a glaze of the color I wanted for the highlight. I had to play around with it a bit to get the right value of highlight, but it did take care of the problem and did not change the color, such as red to a chalky pink. I've used this same technique for WS Oils.
Judy
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