I can't figure out how to simply add white to a color in Sketch. In Illustrator this is as simple as making it a spot color and dragging the slider in the Color panel, but Sketch only lets you adjust RGBA and HSBA values.
Do I need to find a plugin to do this?
There are two ways of doing this, depending on whether you want to be able to preserve the original color or not.
Click on the "R G B" label in the color picker, which will toggle the color picker between RGB and HSB:
The result is a lighter version of the color you started with. The advantage to this method is that your original color is preserved as a fill on the object, so it's easy to go back and adjust, and you can blend different colors other than white or even gradients. Note that you can also reorder the fills with drag and drop.