The idea
rgb(255, 99, 71) spells out red, green and blue as numbers from 0 to 255. Add a fourth number — the alpha, from 0 to 1 — and you get rgba(255, 99, 71, 0.5), a translucent version. Set the box's text colour and background with rgb()/rgba().
This level needs color and background-color; opacity is off-limits so you reach for rgba()'s alpha channel instead.