[quote]Hard is a relative term based on your skill level with a graphics editing program such as Photoshop or Paint shop pro.
The more familiar you are with either of these two programs or others like them, the easier repainting an airplane is.
The basics required for repainting are the afore-mentioned graphics editing program (plenty of freeware options available). a freeware program called DXTBMP is also needed for converting texture formats for FS. Lastly you need the texture to modify. Ideally, you want a paint kit, which are blank textures, but if none is available, you can just use what came with the plane.

If you are just starting out, I highly recommend checking this out:
http://www.gjsmith.net/Tutorial/full_tutorial.htmIt's what I used to get started.
Hope this helps.