Figure the direction on your splotch from dark to light (or light to dark). You’re looking for a defined area which shades with a direction. In the case of this rough green snake, the back is dark and the belly is light. In the case of the Copperhead, the bands are dark on the edge and light towards the center.
Choose threads. It doesn’t matter how many different thread colors you have. You need at least two.
Start on the outer edge with either the edge color. Let’s call it thread color one. Take two strands of thread one, stitch along the edge of the defined space. This is row one. It uses 1,1.
Once a single ring is done, thread a strand of the edge (number one) and a strand of the next lighter (number two) thread onto your needle. Stitch along the defined space with this mix. This is row two. It uses 1,2.
Once the second ring is done, use two strands of the thread number two. Stitch a ring around the defined space. This is row three. It uses 2,2.
Thread a strand of thread number two and thread number three onto your needle. Stitch a ring around the defined space with this mix. This is row four. It uses 2,3.
Once a third ring is done, use two strands of thread number three. Stitch a ring around the defined space. This is row five. It uses 3,3.
See now this goes? Keep going until there is no space left for threads.
If you have only a few colors, and a large space, you can do multiple rings, just stitching around and around the defined color space. The green snake has three rows of dark green, two rows of light green, and one row of mixed.
It takes some practice to get it to come out even. But it’s easier this way than charting out actual color changes, and it looks better too.