The lace in a watercolor painting isn't actually painted. Painting the space around it allows the unpainted paper be the white of the lace. I start with a sketch from a photograph I've taken of the lace I want to illustrate. My final sketch is then transferred in pencil to the watercolor paper with the help of a light table. OK, so now that the prep is finished, here's the fun part!
2nd - Paint the shadows on the lace, keeping in mind the direction of light. Doing this now keeps the color in the open spaces from smearing later. Perfection isn't needed, have fun letting the paint flow. I used a mixture of cobalt blue and burnt sienna.
3rd - With a tiny paint brush, carefully drop paint into the open spaces of the lace, creating some variation in the darkness of the paint, allowing the paint to remain darker at one edge where a shadow may occur.
This is from a new card I'm creating. Painting the lace, or rather the negative space around the lace, is my favorite part of the painting. I hope this example has encouraged you to give it a try. And remember, this type of watercolor painting can't be rushed. If the surface still feels cool, it's not dry yet, painting over it will cause it to be muddy. Instead, put the kettle on for a fresh cup of tea, fold the laundry, feed the cat, whatever needs attention (this is the time I force myself to get chores done because I'd rather keep painting).
If you have any questions, let me know, I'll be happy to help.
Kindest regards, Kimberly