Re: Mixing a good Dark Green in watercolor
To me, 'strong dark green' means dark and saturated. Sap Green, applied thickly (low dilution) over white is definitely what I would call a strong dark green. But Sap Green is slightly yellowish-green in hue; might not be the pine green you want. Adding Ultramarine Blue (which you tried) will pull it towards blue but will slightly grey it. Either of your PV19 (whichever is darker) will darken your Sap green - but will also blacken/grey/neutralize it - which may be why you didn't like the results.
Try adding a bit of any of the colors you have to your Sap Green - that might get the pine green you're aiming for. A lot of artists use a brown to darken their greens. Dark, transparent colors will result in deeper, darker mixes.
Other than using Sap Green, your best bet might be PY65 + a small amount of Phthalo Blue; add a tiny bit at a time until you get a green of the right yellow/blue balance you want.
Making color mixing scales for future reference is an invaluable way to get a feel for the vast range of greens in the world, and how to mix them. For example compare the greens you get from an orange-yellow + greenish blue versus lemony yellow + violet-blue.
Color is the most important element in painting - except for everything else