Ah, thanks - that's the eye-dropper tool for sampling colour from the canvas. It doesn't affect the wetness of the paint applied, but it does change the colour of the paint applied.
To sample colour with the eye-dropper, select tool, then click anywhere in the canvas (or in reference images) to select the colour.
Here's a tip for you: If you're using one of the painting tools, you can press the <alt> key on Windows, or the <option> key on OSX, and click to sample colour without having to switch to the eye-dropper tool.
If you like, you can set aside a small portion of your canvas for colour-mixing, and sample from there with the eye-dropper while you work. Or capture that section of the canvas using the menu option in the colour picker for creating custom colour pickers and use that as a source of colour.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"