Format for recording RGB color in MySQL React client is sending color to my server with this format
 {"r":1 "g":8 "b":5 "a":2}, for instance.

I'm not sure what is the best way to record color in MySQL.

I don't want to use this data in server. Just I want to send it to the client with the same format.
If "you want to send it to the client with the same format", then it is reasonable to treat it as "opaque".

That is, store it as a string with no meaning or structure that you care about.  And it does not matter (within limits) what datatype you use.  VARCHAR(99), TEXT, BLOB -- any of these would suffice.