Here’s the description from the styleguide:
Site colors are used for collaboration. A site is another collaborator
These classes only set the background color, no other styles.
You can also use LESS variables @ui-site-# in your plugins where # is a number between 1 and 5.
Site colors will always be in the color progression you see here.
I’m not sure how to interpret this description but I haven’t seen much use of these classes so you should be fine using them for now.
If it’s for a package it makes sense to rely on theses classes/colors, but if it’s just for your use only (i.e. through the init script) maybe you can use some other classes/colors to make sure you’ll not get polluted by other uses of these classes.