Basically there are 4 ways to darken a color (or create a shadow color), which work for all colors, not only Sap Green. Each of the four methods will give you different results and it’s a good idea to test them and see which one you like the most. Here’s a quick overview:
Use a darker tone of the same color (thicker consistency) (in this case a thicker consistency of Sap Green);
Use an analogous color which has a wider tonal value range and the hue is generally darker (in this case I would use Perylene Green Pbk31);
Use a complementary color (a color wheel may come in handy) (in this case I would use Quinacridone Purple PV55);
Use a neutral color like Neutral Tint, Payne’s Gray or Black – I think this is the easiest and the quickest way to darken and mute down the color at the same time. A neutral color is a mix of two complementary colors. One of those great mixes is a combination of Ultramarine Blue PB29 with Burnt Sienna PR101 (or any other blue with brown), which makes a neutral grey (or black if more concentrated). If you have a color you’d like to darken (or create a shadow of it), let’s say it’s Permanent Rose PV19, you can first add Ultramarine Blue to it and then Burnt Sienna to balance the blue. It’s like if you added a neutral grey to Permanent Rose. I hope this makes sense. If the resulting color is still not dark enough then replace Ultramarine Blue with Payne’s Gray. Payne’s Gray has a much wider tonal value range than Ultramarine and you can take any color with it really dark (and balance with Burnt Sienna if needed).
In my school I have a very comprehensive and in-depth lesson on color theory.