Probably just the particular plaster you bought. Plaster intended for uses where it will be painted or covered with other plaster is not too particular as to color.

There certainly is pure white plaster available, don't know what your local sources might or might not have, or how far from local you might have to go to find it.

The white wall appears to be painted, which is another way to get there.

Art supply store might be one location to try for plaster specifically intended to be white as cast. 

On this side of the pond, "Plaster of Paris" seems to be pretty consistently white (it also sets relatively fast, which can be good or bad depending on the project, and is usually higher priced.) I don't know if that term is used in the UK or if means the same thing if it is.

*Incidentally, all the plaster manufacturer mixing instructions I have read mix a specific weight of plaster to a volume or weight of water, NOT a volume of powder to a volume of water, but that has no bearing on the **color** with clean water.*