[colors] List of ANSI color escape sequences

How about:

ECMA-48 - Control Functions for Coded Character Sets, 5th edition (June 1991) - A standard defining the color control codes, that is apparently supported also by xterm.

SGR 38 and 48 were originally reserved by ECMA-48, but were fleshed out a few years later in a joint ITU, IEC, and ISO standard, which comes in several parts and which (amongst a whole lot of other things) documents the SGR 38/48 control sequences for direct colour and indexed colour:

There's a column for xterm in this table on the Wikipedia page for ANSI escape codes