I began painting on silk some years ago, using spirit-based gutta resist and steam-fix dyes directly onto the white silk surface. This produced designs where each individual coloured element was defined by a line, produced because it was where the gutta had been drawn to separate the dyes.
Following are several different techniques you can use to achieve desired patterns or markings.
Blending - done by merging together two or more similar or dissimilar colours by brushing over where the two colours meet to make a smooth transition.

Salt - on a wet painted area you can either spinkle over varing sized salt or use salt water to achieve a mottled effect.
Wet on Dry - by painting wet dye next to an already dried area, they will meet and form a hard, wavy line. You can do this same technique by putting wet on top of dry areas.
Panting with water - you can use clean water as per the wet against dry technique, sprinkle it over an area, or use an eye dropper, etc.
Splattering - achieved by splattering several colours on top and next to each other in a large area so they can merge and create a colourful area.
Tie and Dye - this has become a favourite method for me. Paint large patches of colour, eg. stripes of light and dark colours. Let dry. Twist and knot into a tight ball then boil. You are then ready to experience the wonder of colours.
I use Sweetwood Silks steam-fix dyes and spirit based gutta for all my silk painting work.