Although I recognise that they're not the same, the yellow berries are just like the more common red ones, but why are there yellow ones ?
For example, I've never come across purple ones.
Well, on a chemical and physical scale, the color of berries depends on the molecules produced by the plant and concentrated in the berries, which can be for a veriety of reasons: to attract or deter animals, camouflage, protection from the elements or microorganisms...These molecules absorb certain wavelengths and reflect others (the latter ones you detect as color).
Carotenoids are a group of molecules (organic pigments) produced by plants that reflect mainly yellow and red wavelengths.
Anthocyanins in contrast reflect red and blue wavelengths. They are 'rarer' than carotenoids.
These molecules may be present simultaneously, but the concentration of said molecules defines the ultimate visible color.
Leaves contain LOTS of chlorophyll to maximize their photosynthetic potential and this molecule reflects most of the green light. They also contain carotenoids for various reasons, but in vastly smaller concentrations. Thus the reflected light from the carotenoids is outshone by that of the chlorophyll, thus leaves appear green to you and me.
This scenario is visualized dramatically in autumn as leaves turn from green to yellow, orange, red, brown or purple: The plant draws the chlorophyll molecules out from the leaves into its woody tissue as reserves during winter, leaving the carotenoids behind. The latter's relative concentration in the leaves increases, and so does its portion in the reflected light.
So why have you not come across purple berries? Depends if you're talking about general berries/fruits...There are LOTS of examples as given by people before me.
Or if you're talking about these specific plant species...They simply do not have the genetic ability to produce anthocyanins - either in great enough quantities or at all - to produce purple berries. Maybe one day someone will breed one though...