Get the right series resistor for your LED — exact value, nearest standard, and watts.
R = (Vsupply − Vforward) ÷ I. With 5V supply, a 2V LED at 20mA needs (5−2)/0.02 = 150Ω. Use the next-higher standard value to be safe. Typical forward voltages: red ~1.8–2.2V, green ~2.0–3.0V, blue/white ~3.0–3.4V.