PowerPoint automatically generates the tints and shades that populate the theme color gallery. You cannot manually select your own colors for these swatches. The tints and shades are based on the values you specify for the theme colors. If your theme colors are overly saturated, you can end up with tints that are practically screaming.
Fortunately, there is a way to tone down those tints by tweaking the saturation and other values of your theme color(s). Want to learn how? Read this tutorial that I wrote with Geetesh Bajaj over at Indezine.com.