Hand painting silk is similar to using watercolour on paper when you apply background washes that diffuse into each other. Dye on silk flows and pigment is dispersed across the cloth. The nature of concentrated dyes radiating out across the silk often reminds me of some of those simple chromatography experiments we did in school chemistry.
One of the principle differences with dyes on silk as opposed to watercolours on paper is that the moment a line of resist is drawn on silk a barrier is created and the colour is then clearly contained and defined. Most commonly these barriers, these resist lines are gutta (a form of liquid rubber) or sometimes artisans use hot wax. The design can be simply drawn out and then coloured in with dye.
Of course, you can use thin (fine nibs) and thick lines (broad paint brushes).
The resist can also be coloured with dye and painted onto the silk.
Antidiffusant can also be sprayed on to the fabric before painting which makes the silk more like paper or canvas and allows a more painterly effect.
I think more interesting silk work is achieved when a variety of methods are used for one piece. Thin and thick resists, different coloured resists and some dyes kept pure and enclosed whilst in other areas the colours are allowed to flow and bleed into each other.