IPL will work better on darker hairs. This is because darker colours absorb more light hence and so there is more chance of light traveling to the root of a dark hair in order to attack the follicle. Conversely, lighter colours absorb less light reflecting most light away, this makes it extremely difficult to treat white hair.
The purpose of IPL is to transfer light down the hair stem and into the follicle without affecting the surrounding tissue. Coarse hair is thicker and can absorb more light. Fine hair is difficult to target and absorbs less light energy. Coarse Dark hair will work well with IPL absorbing most of all light into the follicles. Fine White hair will barely work at all.
One more thing to consider is skin colour. Because dark colours absorb light well dark skin will absorb light too, heating up. Conversely light skin will reflect light away, staying cool.