The easiest and most realistic way to get a dirt effect is to spatter the floor
- right onto the black! brushing color on will look "painted" - spattering,
especially with varying degrees of droplet size, mist, etc., looks to real to
have been painted. There are several methods:
1) flicking a loaded paintbrush gives you a huge variety of droplet sizes, but
is messy for the "flicker" - and anyone else nearby. I genarally use this
sparingly after using other techniques;
2) using diluted paint in a garden hand-pump sprayer works well to blend color
and give the surface a naturally variegated look. It can alo be used to force
lighting and shadowing effects Cool!;
3) or get yourself a plastic laundry spray bottle, and set the output to
several different settings to get a variety of droplet sizes and mists.
Generally, I find that by using all THREE together, I get the most natural
look.
Important Hint: carefully mask off everything (and one) that you don't want
painted. Especially for the brush flicking technique.
Good Luck