You could try this: Lay out a tiny blob of your Bright Blue. To this, add just about one-third as much white to lighten the mix. Restated, 3 parts Blue, one part White. This may work on its own, or be too colourful. If too colourful, then add just a pencil dot of your black to knock down the colourfullness of the mix. This, if it's close, you could "scumble" as a thin layer over a very dry layer of the Rocks/snow layer you have previously painted. Your set is matte - so somewhat opaque/chaulky - this is why the overpainting layer for the shadow should be very thin so the underlayer shows through. Let it dry, evaluate, and possible layer again? Just a thought for a strategy.
It is possible that your blue is green enough in bias that it won't match the photo exactly as well as you would like. I would just think of working with your own mixes, and how they work together for contrasts one mix against another, and not worry too much if it's not an exact match to the photo.So, the end result with the paint set you have could be seen as if the sun were "warmer" casting more of a warm glow onto the mountain, rather than the mid day "cold" blue of the photo. It's a very subtle difference for you painting, though I've added the shift a bit more strongly (just for illustrative purposes) here: