Version 2

I think this is the solution. I've tested it in the following cases:

• Client with colored name joins a 6.3 CTF server that later changes to Battle
• Client with colored name joins a 6.3 Battle server that later changes to CTF
• Client with non-colored name joins a 6.3 CTF server that later changes to Battle
• Client with non-colored name joins a 6.3 Battle server that later changes to CTF

...all successfully. The only case that doesn't work is:

• Client with colored name joins a 6.3 CTF server with /joinersspectate enabled that later changes to Battle

But as far as I can tell, this was always broken. It's just as much a problem for 6.3 clients as for 6.5 clients. If someone hosts in 6.4 or 6.5, it'll work, because 6.4 fixed that issue on the server side, but if someone hosts in 6.3, I haven't found a way for the client to convince the server to fix itself.

I haven't tested extensively with gamemodes other than Battle and CTF, because I'm assuming that Race, Treasure Hunt, SP, and Coop all work the same way as Battle, and everything else works the same way as CTF. But that's an untested assumption.