i just bought a prop for my new 4 stroke engine, i checked and found the prop is heavy on one side. can they be balanced or should i take it back to LHS. I dont want to trim the prop on one side, but thought about dipping the light end in some paint as the tips are painted white anyway.

Any help appreciated.

I hope you have a good quality balancer that you bought from the hobby store - they make things a lot faster and simpler. All props should be balanced by the user and trying to return them will probably get you laughed out of the store, because most fliers never balance their props. Here's how to do it:
If you need to remove weight from a heavy tip, DO NOT shorten that tip. Lightly sand the backside of that blade to remove some weight, never the frontside of the prop, nor the leading or trailing edges. The farther out toward the tip you remove (or add) weight, the better, as a little at the tip equals a lot more, farther inboard. Sand a little, check the balance, sand a little more. I never use anything more coarse than 120 grit, usually using 240 grit.
You can also add weight to the lighter blade by LIGHTLY spraying it with some paint. Again, spray the backside of the blade. Many people like to use a clear spray paint, but I like to use a color so I can easily see which prop I have balanced. Give the paint a couple of minutes to properly dry after each spraying, as any remaining solvent in the paint will give you a false reading. And take it easy, a little goes a long way.

btw, they don't need to be "perfect", just well balanced.

Another tip I picked up from someone else on RCU is to use clear nail polish to balance the light side. Also, there are inexpensive balancers like this one which I purchased:
http://www3.towerhobbies.com/cgi-bin...?&I=LXK247&P=7

I use clean nail polish on the back of the light blade and it works great. Just recheck the balance when the stuff is dry to account for evaporation. Changing the shape of an airfoil (prop) is not a good idea. Sanding the back of the heavy side works, but I believe that removing material = removing strength.

I use an X-acto blade to scrape the FRONT of a prop until I get it balanced after I use 220 grit sandpaper around the edges to smooth it out. It usually takes only a tiny bit of scraping.