In Lightroom's Library, you may assign color labels to your images – Red, Yellow, Green, Blue or Purple – to communicate various purposes. The colors themselves are abstract, and each photographer will assign their own significance to each. In the Web module, CE3 engines are capable of reading your color assignments from the Library and displaying color labels on images in your gallery:
This feature may be enabled or disabled, and colors may be changed or rendered in less garish shades via the UI:
The gallery will even display your Color Label Set's text via tooltips. See the tooltip displaying over the green label here (the mouse cursor is invisible):
Admittedly, the word “Green” is fairly pointless in this context. Fortunately, Lightroom allows you to create your own Color Label Sets. To do that, you will need to be in the Library. Then plunged through the menu as shown below.
Whatever labels you assign your colors, the Web gallery will pick up when creating your gallery.