Just so you know, Thunderbird just gives an item the colour of the first selected tag.
Thunderbird does have another nice touch: the tags to which colours are assigned are displayed in that colour. If Zotero could do that in the tag selector (and possibly the Tags pane in the right panel) it would make for a nice visual reminder of what's what.
devs have said that colored tags will be implemented and most of the code, as I understand, is already included in the 3.0b.
Upload image file with the file dialog or by dragging and dropping images onto the dashed region