Rose gold, sometimes refered to as pink gold, is made by increasing the amount of copper mixed with pure gold.  As there are strict gold standards the amount of pure gold remains constant while the mix of alloys is changed to allow for the increase in copper.  

For example 18ct gold is made of 75% pure gold, with the remaining a mixture of silver, copper, zinc, and palladium.  In order to make 18ct rose gold the amount of copper is increased within the 25% mix, as the 75% pure gold must remain constant in order to achieve the 18ct gold standard.

Traditionally rose gold is found in jewellery dating from the Victorian era.  Modern rose and pink gold have a different colour to antique rose gold due to the patina of antique jewellery and also because more copper is used in modern jewellery.