Satin stitch is great to fill in areas of colour. It’s best to use just one or two strands of thread at a time, and take your time to ensure these don’t twist as you stitch so they lay flat. Try not to use stitches that are longer than your needle, otherwise these can sag. If your area to fill is large, try long and short stitch or brick stitch instead.
I sometimes find it easier to outline the area to fill in back stitch before I start, then go OVER this when doing my satin stitches.
Here's how ya do it!
Bring needle up at point A and re-insert at point B, pulling thread all the way through to the back, making one long stitch.
Bring needle up at point C and then down at point D pulling the thread all the way through to the back. It can be tempting to start the next stitch next to point B, but do go up to point C instead as otherwise your fabric can pucker.
Repeat these steps until the area is filled in.