I'm surprised we don't have a ColecoVision tag. I would like to request that be added (or, how can I add it myself?).
I would also like to add the SG-1000 but I'm up for suggestions on that. At least a generic "Sega" tag would be nice.
If you have sufficient reputation, you can create tags yourself.
If you have a question to add it to, you can just type it in the tag box to add it to the site. However, make sure that it fits the style of existing tags (e.g. hyphen between model name and version number).