Concrete can by coloured or dyed with a limited palette of several colors. Because the portland cement is caustic, some pigments that work well in paint media are attacked by the causticity, resulting in colour break-down over time. To be certain, you must use pigments that have been proven to last when mixed with concrete. Green is less common and more expensive. Blue is even more expensive.
Most colours seem to be derived from iron oxide or cobalt.
The colours typically come in a powder form, or mixed with water as a paste. Often the main body of concrete is mixed without the dye, and then a mix of cement and pigment used as a top coat. This gives you the strength, with a deeper colour on the surface. Acid stains work by painting the acid stain on the surface. The acid creates porosity and the stain penetrates.