You can buy chalkboard paint in green or black, but why limit yourself to those scholastic-y colours? Maybe you want to make a chalkboard wall in the kitchen for a family communication center, or maybe you're making a kids' art table like this...

...and it would be fun to use a colour other than black or green.  

So get a quart size can of flat paint in any shade you like (I used Olympic Premium Zero VOC because it dries in an hour, has no odour and carries a lifetime warranty) and pour the amount of paint you'll need into a separate container.  Then add unsanded grout powder (available in the tile aisle) with the ratio of 1 rounded tablespoon of unsanded grout to 1 cup of flat paint.  Stir it up well and then roll out 2 or 3 coats on the surface you want to transform into a chalkboard.  You can use a hair dryer to speed up the drying time if you're rushed.  Cool eh?