You can wash quilts that have been made with watercolor pencils, but they need to be heat set first. If you color your blocks and heat set, then soak them in water before piecing and quilting. I have not found any color to come out.
I'm working with watercolor pencils (Michaels). You can use water to take the color from the surface of the fabric down into the fibers. However, water can make it bleed outside the lines. You work from the middle and push the dampness to the outside. Reading these posts I find that fabric medium works, but I haven't tried that.
Just this week I learned that clear aloe vera gel works great and there is no bleeding. I used it and like it. It dries quickly (5 minutes or less) just like water. You can heat set it before it is dry by using a pressing cloth.
I haven't tried painting or using watercolor pencils AFTER quilting. I would question heat setting after quilting as it could damage the loft especially if using poly batting. I do use wool in my quilts which gives loft but doesn't melt when pressed.