Let's say this color is on your screen:
(click the rainbow button to find other colors).
Perhaps that color reminds you of cheese fries. Yum.
Click the "Add Tags" button to add a tag.
Tag added! (actually 2 tags in this case).
That's pretty much it.
Click "More" to see all tags ever added or removed from a color or scheme.
From this view, you can delete tags or re-add them. Tag history is preserved (tags are never deleted for good).
Color schemes are much the same. You can add and remove colors (click a color to remove it).
Changing a scheme's colors creates a brand new scheme. The new scheme won't get "saved" until you add a tag.