You really shouldn't be driving LED's directly from pins if they are high-brightness types.. you may want to use a switching transistor so that you can provide more current and potentially add more LED's...
Also, I've found that the "whiteness" can be helped by PWM blending of white with the RGB.. allowing for more light output. I typically put all the LED's (RGB+W) inside a semi-translucent shell, which helps blend the colors. Ping Pong balls work well for this...