You can give nicknames (for instance, PageBackgroundColor rather than #999900) to assets only in the Favorites list. The Site list retains their real filenames (or values, in the case of colors and URLs).
Right-click (Windows) or Control-click (Macintosh) the asset’s name or icon in the Assets panel, then select Edit Nickname.
Click the asset’s name once, pause, then click it again. (Do not double-click; double-clicking opens the item for editing.)