Welcome to BA!
First point, Post a blend file or images of what you are doing please, we can help you much better if you do. If you cannot post here yet until you have your required 10 posts to move off moderation - post to pasteall.org and post the link here.
You will need to use a particle system to represent the paint being sprayed, not an easy task for a beginner I must say, perhaps you should try something a little easier to start with! Then you will need to play around with opacity of the material for the surface being sprayed, i.e. animate the opacity levels as the paint is applied. For this you need to decide whether you are going to use Cycles render or not, personally i would. You can simulate a surface changing colour by having two colour feeds to your material and gradually animate the change from one to the other. The paint itself is going to be a huge headache for you until you are pretty familiar with Blender.