Thanks for the details. Here's how it works.
All of the items in the presets panel are collections of settings, they're not individual brushes. So, when you select a preset what you're doing is selecting a specific set of settings for that tool .e.g. 'just water' is a preset for the watercolor brush. You will notice if you have both the presets panel and settings panel open at the same time that choosing a different preset just changes the values in the settings panel.
If you select a preset, then change a setting in the settings panel, the preset is no longer highlighted as you're no longer using that preset ( due to changing the settings ).
The 'thinners' setting controls the amount of paint vs water on your watercolour brush. 'Just water' uses 100% thinners in order to paint just water. Note that there is never absolutely no color at maximum thinners, so you will always get a small amount of the currently selected colour. To avoid this, set your colour to white. White isn't a pigment with watercolour in ArtRage, so no colour will be added. Reducing the thinners value will add color again, you're no longer using the 'just water' preset, but a watercolor brush with the thinners value you just selected.
I hope this helps.
Resident Bug-Hunter and Support Guy