Hi - "900" is not a common gold standard mark. On rare occasions you may see something different. The odd ones can easily be calculated using following:

1 karat gold is 1 part in 24 gold.

So for instance 22ct or 22K is 22 parts in 24 of pure gold or 22/24ths or if you multiply 22/24 X 1000 you get 916 (916 parts per 1000) or 91.6% gold.

If you see a number, for example 625, divide it by 1000 and multiply by 24, it will give you the carat. 625 / 1000 x 24 = 15ct.

The carat does NOT change because of the colour. The amount of gold is the same. Gold comes in many colours depending on the other metals used in the alloy.