>Do we need a 'livery' tag?

I don't know. 

>It seems unlikely that this tag will be very useful.

I'll list some questions where it *might or might not* apply. There may be a few more out there.

>Are there roughly similar ones already?

I found a few but not all using the [tag:space-art] tag. We don't have a "markings" or "symbol" or "logo" tag, 

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Note, I don't know how many of these describe markings that would count as "livery".

 - https://space.stackexchange.com/q/6352/12102
 - https://space.stackexchange.com/q/25314/12102
 - https://space.stackexchange.com/q/44173/12102
 - https://space.stackexchange.com/q/28424/12102
 - https://space.stackexchange.com/q/21586/12102
 - https://space.stackexchange.com/q/37738/12102
 - https://space.stackexchange.com/q/38975/12102
 - https://space.stackexchange.com/q/41458/12102