Resistors, what do I need for my 3V LEDS?
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simulate this circuit &ndash; Schematic created using CircuitLab
Both cool white and warm white are similar - the only difference is the phosphor coating the underlying blue LED. So both types drop a similar 3 volts.
For the single LED (D5), the resistor must make up the difference (9V - 3V)=6V. With 15mA flowing through both resistor and LED, the resistor should be 6V/0.015 ohms. That's 400 ohms. Nearest standard value is 390 ohms. You're using 1% resistors? That's OK, find one near 400 ohms.
For the two-diode string, both LEDS in series take 6V of the 9V battery. That leaves 3V for the resistor....3V/0.015 gives 200 ohms. Use a resistor near this value - a smaller resistor gives more current, and those two LEDS will be a bit brighter.