First, difference between karat and carat
Karats ("kt" for short) is a measure of the purity of gold. Don't be confused with carats ("ct" short) - a measure of weight of precious stones.
Meaning of a karat
Karat is used as an identifier, and all jewelry made of real gold will have a karat stamp. Although percentage of precious metal present in jewelry can be any number, just a select few standards are commonly used.
The commonly used karats are:
10k or 417- 41.7% of pure gold
14k or 585 - 58.5% of pure gold
18k or 750 - 75% of pure gold
24k or 999 - 100% of pure gold
These karats apply to any color of gold - meaning that 14k yellow and 14k rose gold will have same percentage of gold, 58.5%. And, here's an explanation on why gold can have different colors.
Karat stamp (10k,...