It seems that the only way to color rate a group of images the same color is through the metadata template. Thee is no way to "right click" and set the same color on a bunch of highlighted images
From the main menu: Image->Set Color Class of Photos->choose from sub-menu of colors
Make note of the keyboard shortcuts and you can do them directly. Or from the color class on the thumbnail, hold down the "Modifier Key" (Shift on Windows, Option on macOS) and choose a color from the popup menu. The entire selection of photos will be changed to that color.
HTH,
-Kirk