Works only in Team chat and it will work in global chat only while you're dead


Use 
XXXXX before the message + 6 digit color code + your message without any spaces to make it colored
use  before the message to reset it to default color



XXXXXFF4040ok -- red colored "ok"

XXXXXFF4040ok but why? -- "why" will be default color, and everything else is red

Here's the 6 digit color codes for TF2 qualities

FF4040 - RED Team
99CCFF - BLU Team
8650AC - Unusual
AA0000 - Collectors
CF6A32 - Strange
38F3AB - Haunted
FFD700 - Unique
B2B2B2 - Normal (quality, not the default color)
4D7455 - Genuine
70B04A - Self-Made
A50F79 - Valve

to make it look real, you should write a short message (example: gl, ok, etc) and after that (A LOT) of spaces to make the colored message be on the SECOND chat line
You should test on your private server the amount of spaces which will work for you because it's different for everyone (username length rules)


example:
gl                                                    
XXXXXFF4040ManDefense 
has unboxed: 
XXXXX8650ACUnusual Team Captain
 
put the amount of spaces that works for you