Well SAI is software that works best with a tablet, but without a tablet you can change the settings a bit.
Go to Others > Options, make sure the mouse is selected there and play around with the drift. That'll make your mouse work a bit like a tablet with pen pressure, so search for the right amount of drift untill it makes the colors flow into eachother nicely ^^
I had water brush not just water color. The next time I used Sai, it was gone. I uninstalled and reinstalled many times but it is still gone. Anyone know how to get it back. I need it for backgrounds and blending.