These are all the corresponding color codes that work in chat and for mob names.
Alt code for §: 0167
This method doesn’t work with sign text, for that you’ll need to use dye and right click the text directly on the sign.
§e Red
§2 Magenta
§6 Pink
§a Purple
§b Blue
§9 Aqua
§3 Cyan
§d Dark Green
§5 Green
§4 Yellow
§1 Gold
§c Brown Bricks
§0 White
§8 Gray
§7 Dark Gray
§f Black
§k
§o Italic
§l Bold
§n Underline
§p Strikethrough