Between 18-36 months, you can start introducing colors to your toddler 1 color at a time.
 
Have a color of the week.  
 
A few activities to get you started....
 
Sorting Colors- you can use pegs, blocks, pom-poms, etc. 
 
Matching Colors- print 2 copies of the "Free Printable Activity Page", leave one as it is and cut the other one into 6 squares to make a set of cards.  
Now let your child match each color.
 
When reading books to your child, point to the color of the week and repeat the color multiple times during the reading.