Why do We Cure & Seal Decorative Concrete?
One problem with colored concrete that plain gray doesn't worry about much is efflorescence (our baby has a skin disease!). Efflorescence is a powdery white material that forms on the surface of the concrete and that is caused by calcium hydroxide which is naturally in the concrete after it sets. As water moves out of the concrete it carriers with it some of this calcium hydroxide, which then reacts with carbon dioxide in the air to become calcium carbonate. On plain gray concrete, you really can't see efflorescence, and even if you can, typically no one worries about it. It doesn't affect the concrete's strength. On colored concrete, if you are careful about mix design, and get a cure & seal down as soon as possible, you can eliminate some of the danger of efflorescence.
The other primary reasons for sealing decorative concrete are to bring out the color, make the surface shiny, and protect it from stains. For details on sealers, read The Concrete Network's Guide to Buying Concrete Sealers.
Find Manufacturers: Concrete Cures
Find Local Suppliers: Decorative Concrete Stores