Assuming you're using Photoshop, you can switch from the brush (B) to the color-picker tool by holding down the ALT key.
This means you can paint something, then grab the local color as you go, and keep painting with that new color to help with your accuracy and blending.
Beyond that, pen pressure and control take a long time to master. Think of it as learning to play an instrument - there's a LOT of time and practice required!
