256 colors.

ESC[ 38;5;⟨n⟩ m Select foreground color
ESC[ 48;5;⟨n⟩ m Select background color

where n is:

  0-  7:  standard colors (as in ESC [ 30–37 m)
  8- 15:  high intensity colors (as in ESC [ 90–97 m)
 16-231:  6 × 6 × 6 cube (216 colors): 16 + 36 × r + 6 × g + b (0 ≤ r, g, b ≤ 5)
232-255:  grayscale from black to white in 24 steps

where:

• 0 is BLACK.
• 1 is RED.
• 2 is GREEN.
• 3 is YELLOW.
• 4 is BLUE.
• 5 is MAGENTA.
• 6 is CYAN.
• 7 is WHITE.

echo -e "\033[38;5;1mThis is in RED"

echo -e "\033[38;5;9mThis is in BRIGHT RED"

for (( i = 0; i < 16; i++ )); do echo -e "\033[38;5;"$i"mHello"; done