The decals mod allows you to easily add decals with a single function.
Decals can be placed in any orientation. Cycle through the different colors by right or left clicking.
decals.register_decal(name, desc)
the name variable is the internal name of the node, also it is the name of the texture.
the desc is the node description.
example:
decals.register_decal('sample', 'This is a sample decal')
The texture in this example should be named decals_sample.png