Which tank are your seahorses in? Your 24g nano, or your 75g reef? If they are in the 24g, then the 2 you have is about enough. Adding more will strain the water quality.
Also, there are lots and lots of other concerns regarding adding new seahorses to an already existing tank - color is not one of them. Seahorses can and will change color to suit themselves, and just because you buy a horse of a certain color, doesn't mean it will stay that color.
First of all, how long have you had your 2 females? Where did you get them from? Are they WC or CB? Where will the new horse(s) come from? Are they WC or CB? Are they all eating frozen mysis well? Do you have any other tankmates?
These are just a few of the many questions which should be answered BEFORE you even consider adding more horses. The fact that you had to ask if mixing colors was OK tells me there is more reading you need to do to learn about seahorses. The fact that you did ask before you purchased lets me know that you are interested enough and care enough to find out.
There can be some very serious consequences to adding seahorses to an already established tank. The worst is that you can wipe out your entire herd, and cause an environment which is no longer suitable for any seahorses. As long as you know and are willing to take the risks, then carry on and good luck to you, but just as an FYI, most inexperienced keepers do not fare well when they add a horse from here and another from there.
My advice to you would be to enjoy the two horses you currently have. Learn about their needs, do some more reading, find out about raising reidi babies, THEN consider adding a male to the tank. Reidi babies are extremely hard to raise, and require a very elaborate nursery system. If you add a male with your two females, you will eventually get babies, probably between 500-1000 at a time, as often as every 19 days. Most, if not all, of them will die. This can be very discouraging to a new seahorse keeper.
Besides here at ReefCentral, there are a couple of other sites which have a ton of information. They are
www.seahorse.org and
www.syngnathid.org. Both deal only with seahorses and pipefish, and are full of information.
Good luck to you, and keep asking questions!