Humidity passing through a cement roof can cause ceiling paint to blister and peel. Not so with colored cement, which sticks like crazy to cement, even wet. With luck, an interior cement "paint" job is a lifetime paint job.
On the outside, cement paint helps seal cracks.
I use a pressure sprayer for preliminary cleaning.
One interesting advantage to using cement as paint is that spiders appear to not like raw cement. There are very few spider webs where surfaces are painted with cement. If you paint them with house paint, the spiders come. Some insects (butterflies?) taste through their feet, I think. Although not insects, that may explain why spiders don't like to hang out on raw cement. Whatever the reason, fewer spiders in the house is good. Fewer cobwebs to clean results in less time spent cleaning.
Step 1: Materials
I use both regular gray Portland cement and white cement, depending on the colors I am after. For darker colors, I use gray cement. The more pigment you use, the more intense the colors will be. The cement is the binder, though, so an excess of pigment could mean not enough binder and result in a chalky surface.
Pigments for cement come in liquid and powder form. I prefer the dry powder form, since I can mix the pigments with cement and store them for later use. (Water in the liquid pigments would harden up the cement.) One way to mix the different powders is to put them in a closed container, such as a plastic bucket with lid, and shake them.
Unlike the powdered pigments available from some art supply stores, the pigments from hardware stores are more limited in color, and a lot less expensive. Red is sort of a "red oxide", yellow is sort of ochre. There is also a green, blue, and black. With those basic colors, which are not true primary colors, you can mix a range of colors.
This Wikipedia link will tell you way more than you need to know about Portland Cement. http://en.wikipedia.org/wiki/Portland_cement