Painting finished models seems to scare people new to building with cement. It's really not difficult, and will hide the color differences in batches of building blocks.
After first sealing up open seams with a tube of mortar repair, and letting your structure dry thoroughly (overnight will do), spray the entire model with a flat gray paint. It doesn't matter if it is interior or exterior and doesn't need to be any special brand. A $0.99 can of Walmart flat does nicely.
For the next step, you can use a soft, stiff bristled stenciling brush or a rag you can get a good grip on. Get a few inexpensive earth tone bottles of acrylic paint from the craft department and squirt a quarter size amount on a discarded plastic coffee can lid. (I usually start with brown). Dip the ends of the brush in the paint, and wipe it almost dry on a rag.
Brush your model with an upward motion from left to right, 8 o'clock to 2 o'clock. Brushing at an angle keeps the acrylic from nesting between the mortar liens of individual stones.
If you feel brave and want to be creative the first time out, "dirty" stones lightly around the top of doors and windows, especially at the corners (using black). Also, moss or mildew along the ground mixing green and brown, especially where building would be always shaded on -- by the roof, corners, and under sills.
Whatever you try, remember that you can always go back to the start by respray-painting your house gray all over again. Once you're finished, spray with exterior clear flat polyurethane.
