>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. I think there might be one more on identifying specific Saturn I or V launches based on some differences in markings, but I may be mis-remembering.

>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/5964/12102 "Progress rockets I saw have different "liveries", multicolored like this"
 - 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 (it's a <s>boat</s> ship, not a spacecraft)
 - 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
 - https://space.stackexchange.com/q/38211/12102
 - https://space.stackexchange.com/q/51799/12102 (also mentioned in the question)