What make them coloured?

Heart in Diamond offers diamonds in the following colours:

Yellow, orange, green, red, blue, white and black

Yellow and Orange
diamonds contain Nitrogen (N) which makes this colour.  The more Nitrogen in the lattice the more saturated the yellow will become thus beginning to look more orange and even brownish. Orange and Yellow lab-grown diamonds are Type Ib diamonds.

 Blue diamonds
receive their colour from Boron (B). The amount of boron used will influence the saturation of the blue colour.   Blue created diamonds are Type IIb diamonds.  No irradiation is used to create a Blue colour.

 Green and Greenish-Yellow
diamonds are similar to yellow with the difference in additional growth process due to a higher temperature and pressure than used to create orange. No irradiation is used to create a Green colour.

Red and Black diamonds
are created after the growth process has been completed, which is called electron bombardment.  This is an absolutely safe process commonly used in medical practice. The colour is stable and very rich.