I specialise in special, fantasy airline colour schemes that I design myself, instead of using any standard airline liveries. I never use decals. My liveries all come from my vivid imagination (I'm an artist). For me, this is a niche art style--using airliner fuselages as canvases! Fantasy airliner art, I call it!
I usually build the model kit the standard way (as per the instructions in the kit), then paint on the livery that I have designed.
I paint the fuselage and tail the main colour, then use smaller, thinner brushes for smaller details. For lettering and more intricate designs, I use paint markers (markers with paint instead of ink) with chisel-shaped felt tips (these are more precise and controllable). You can find these at most hobby shops where they sell models, paints, and supplies, or ask the dealer where you can find paint markers. They come in several metallic colours (silver, gold) and black. There is also a wide variety of brush-on enamel paints for plastic models available in shops and online; there is just about every colour imaginable. I have been able to use metallic green paints such as Testors' Jade Green Metal Flake for my metallic green fuselage colour scheme (it looks beautiful on an aircraft).
On the fuselage, I paint the airline titles (Smith Airways) with a gold paint marker (over the metallic-green painted fuselage). On the tail, I use both paint marker and brush to "draw" and paint my intricate Celtic-inspired design.
The engines also get a nice paint job--all metallic green on nacelle with thin gold band around front.
To design your own fantasy airliner model paint scheme takes a fertile, vivid imagination--and the eye of an artist. To me, it's more fun and exciting than just using the decals and painting the standard airline colours on the kit.
Let your imagination take hold, whether you are using real paint or the digital equivalent!
I will praise thee; for I am fearfully and wonderfully made... (Psalm 139:14)