The use of which Laser we use depends on the tissue we are trying to target. We use the wavelength of light which that tissue will absorb, heating it up, and thus removing the problem.
In the case of hair, we use a wavelength that is absorbed by the pigment in hair, Melanin. This results in the hair being heated during the treatment, which then heats and destroys the hair root, reducing the amount of hair in that area. Dark hair has a lot of Melanin, so is much easier to treat successfully compared to fair or red hair, which have very little Melanin.
Since the pigment in Skin is also Melanin, carrying out such treatments in Tanned, Black or Brown skin, can also result in the burning of the skin. Obviously extra care must be taken in such patients.
In the case of blood vessels, we use a wavelength that is absorbed by the pigment in blood, which is Haemoglobin. We use a laser for the larger prominent blood vessels, and an IPL machine (Intense Pulsed Light) for the more diffuse smaller veins of the nose, cheeks and chin.