You'll use a color [Hex] code to do the trick. Follow this formula:
/mydoormess <FONT COLOR="color hex">YOUR TEXT HERE
sm_mydoormess <FONT COLOR="color hex">YOUR TEXT HERE

You can also do somthing like this
/mydoormess <FONT COLOR="color hex"><FONT SIZE="45">YOUR TEXT HERE
sm_mydoormess <FONT COLOR="color hex"><FONT SIZE="45">YOUR TEXT HERE

for example
sm_mydoormess <FONT COLOR="#ff0000"><FONT SIZE="45">YOUR TEXT HERE

Would Look like
YOUR TEXT HERE
Note sm_mydoormess goes in cs:go console and /mydoormess goes in chat you have to have your crosshair on the door

CONGRATS YOU NOW HAVE A COLORED DOOR

Thank you. I stickied the thread so it will always remain on top of the board. We'll show this to donors as well and link it from the donate page.

I usually just manually show people how to do it but
THANK YOU PANDA.