Use red for strength.
Yellow for confidence.
Pink for love.
Purple for spirituality. And use a candle that represents her birth month and day so its clear the spell is directed to herself if you dont know what color to use for her birth then use white
The color of the candle is mostly meant to enhance your visualization. If I was doing that spell for myself for example, I would probably use pink or red, just because that's the color I imagine feminine energy being.