Watercolour brush opacity

I'm a new user of Painter X and cannot get consistent operation of the opacity control on watercolor brushes (I was using watercolor - fine bristle). Increasing the opacity (either manually or with the graphics tablet) sometimes darkens the color as I would expect (sampling the color applied at different opacities always gives the same hue value) but other times it changes the colour e.g. if I select greenish yellow on the colour wheel, low opacity gives yellow but this turns to green as the opacity is increased. I have restored the default settings on all brushes but still get the same effect. Any advice would be welcome.