This is a general thread that will help to explore ways and techniques you can use in giving your paint job a custom paint job, as there always seem to be questions on the subject. If you want to see a fantastic method of giving your gun a great camo paint job, check out chad7881's thread here:
I've included a number of general steps to help anyone starting out have to a good place to begin. Feel free to add any information you find helpful or useful.
1) The first step in painting your gun is often the most overlooked: RESEARCH
You need to have an idea of what you are hoping to accomplish, and there is more often than not some sort of image you have in your head. Google search your gun, save lots of pictures of it. Make a folder on your computer with nothing but images you really like of your weapon. The more quality images you can find, the more you can try and replicate or create your own look. A great place to look if you're going after a gun from a film or television show is the IMFDB's website: http://www.imfdb.org/index.php?title=Main_Page They have more info than you can shake a stick at. Check around lots of different sites. The more inspiration you have, the better off your results will be.
2) After you have the image of what you want in your mind, the next step is figuring out how to create it. Ask lots of questions on forums for specific details regarding colors and paint types. Odds are someone can help you out. Read guides like this one. Actively seek out help from the budding communities around you.
3)Paint Types: With airsoft guns, you have two real options of paint: Spray paint and Duracoat. Duracoat is its own beast, and something I have no experience working with, so I couldn't help you out there. Duracoat is often applied to real steel weapons that get much more heated than any airsoft gun ever could, and they are a much thicker type of paint because of it. Spray paints are much more accessible and easier to work with, and what most people end up using anyways.
As far as as paint colors, the sky's the limit. Make sure to use a Matte or Satin paint though, unless you want your gun to look shiny. If you were "gold plating" a gun, however, you would obviously want to go as shiny as possible. Krylon makes a group of "camouflage" colors that are quite nice, including my favorite matte black which I use for the majority of touchups. This is where the research comes in handy. Take in pictures with you of the way you want the gun to look when you buy paint. Visually compare them and see for yourself just how closely they match up.
Also, ALWAYS use some form of clear coat, again matte finish for most projects, for when the paint job is complete. It will save your work, and will help protect your gun.
Now that you have your paints selected and ready, you begin the long, arduous part. With any paint job, the key to results is patience, and this section here will make or break your custom paint job. The first step here is thorough cleaning the gun, however if there are rough edges or surfaces on the gun that are sticking out, use some sand paper to make an even finish before you start cleaning. Warm water, a towel for drying, and a sponge of some sort with a light soap will work wonders. Clean everything you plan on painting, making sure to get off all of the dust and dirt. Any residue left will affect the results of your paint job greatly.
Disassemble any part of your gun that you need to, such as handguards, slides, receivers, barrels, etc. If you are doing a two-tone job, this is a MUST. Section off what you don't want painted by using painter's tape. Painter's tape is a low-adhesive tape that won't hurt your existing finish. You can find this anywhere you get your paints at, and it comes in a variety of different sizes and colors.
5)Setup a good place to paint. The paint cans will list ideal painting and drying conditions that you can work in. Warm air, with moderate humidity and good ventilation is the absolute best. Avoid wind if at all possible. Use clothes hanger wires if possible to place individual pieces up to paint. This will help you pain the entire surface at once. If you aren't able to use hangers, take some wax paper and lay it out on your painting surface. Wax paper won't leave residue or peel off awkwardly like newspaper would. Also, you can buy a roll of it cheaply.
For any can of spray paint, I recommend using a pistol grip spray adapter. It is much easier to get an even, clean finish using one of these than trying to keep a steady amount of pressure on the nozzle. For the $2 you spend on one, you will make this process much easier for yourself.
Apply thin, light coats of paint. Be patient with this process. Do one coat, then let it dry for 24hours before you start the next coat. This will allow the coat to fully dry and seal onto the gun. Make at least 3-5 coats on your gun. Do not rush this process. The more thin coats, the better it will look. More than 5 is not recommended, and even 5 is quite a bit. Take a week to finish the paint job, working on it only about 5-10 minutes each day. You won't feel the need to rush, and you will get a nice set routine after a couple of days of how to do the process.
Once the main paint job is done, you will need to clear coat the gun. Again, about 3 or 4 coats here should suffice. Do not put too many layers of clear coat on, or it will start to either yellow the surface or make it look "frosty". Paint the gun like you would using any other spray paint can. The only difference here is time between coats. You only need about 2 minutes of drying time between coats, so it takes much less time. After the last coat is applied, wait another 24 hours for it to bond and seal.
After the 24 sealing time, take off the painters tape. If there are any places you need to go back and fix, you will notice them immediately. You may also want to add in some detailing bits here. For example on my Sig Pro, I went in with a red paint pen and painted the safety dot red again. After any detailing has been complete, seal it with a last coat of clear coat again, and wait another 24 hours.
Reassemble the gun, and enjoy it. Show it to your friends, family, and take lots of pictures of us here online. We love to see custom guns!