This is a great trick that I learned from someone  and that I use almost daily. To mix colors quickly, first select the vector shape that has the color you want to alter, and then holding down the Ctrl key, click the color you want to mix it with on your color palette. 10% of the color you click is added to the vector shape.  This tip also works with outlines -- instead of clicking, try right-clicking the color.

Provided by Joe Diaz, CorelDRAW  Master, Graphic Designer and Sign Artist