Hi Caleb,
Unfortunately there is no quick key or pressure to help you do that.
But you can certainly change the color and make it darker and lighter.
To do that
1. CLICK on the swatch and click on the little color wheel icon

2. Click on "B" for Brightness(highlighted in Red) within the "HSB" (Hue, Saturation, Brightness) color and drag the slider to your color to a darker or lighter hue (highlighted in green).

To speed it up a little.. I'd recommend you suggest to Adobe that they add a quick key to quickly sample colors (like pressing the Option key in Photoshop). But you can choose your colors and create little swatches like an artist's palette. Then you can press "I" on keyboard to get the eyedropper, select the color swatch you want on your stage (see image below) and then switch back to the brush tool by pressing the "B" key on keyboard. Not the best, but I think it should make it pretty quick.

Hope that helps!
Cheers,
mark