I wanted to add to the discussion about cement pigments:
Integral dyes and pigments for cement are specifically made to
resist the alkalinity of the cement, otherwise they will break down
over time. The cheapest and the most durable are those you commonly
see in coloured concrete blocks: black (grey), brown, brick red,
yellow ochre. These are usually iron oxides. Green and blue are more
expensive and often not as vibrant. Sources of supply: building
centres, masonry supply outlets, sculpture supply outlets.
You can obtain the truest and brightest colours if you use white
Portland cement in the mix rather than the common grey cement. The
colour of the aggregate (sand, stonedust etc.) will also affect the
final colour. As a general rule, do not use more than 10 per cent
pigment powder compared to the weight of the cement or the final
concrete will be weakened. A good chart of the available colour range
is at: http://www.solomoncolors.com/concreteclrs.htm
You can also use acid stains on the surface of the set concrete, but
the colours are not as deep as integral colours. The acid etches the
surface and the pigment stays in the recessed areas.
I have also had success with opaque exterior latex stains, painting
them on the surface, then rubbing the excess off while it's still
wet. This works well on sculpture, but I suspect it would not wear
well on jewellery worn next to the skin.