Mixing in pastel

Pastel is a new medium to me, so I am still experimenting with different techniques. Both paintings were done a week apart, on similar neutral lighter tone paper, and somewhat similar lighting conditions. Both had lighting poking through the clouds, except that the first one was mostly cloudy.
In the first one, for the sun light on the mountain, I mixed the color on the paper, to create this yellow green color.
I find mixing with pastel extremely difficult. I only have about 60 colours, apparently that's plenty for pastel, but mixing on paper is very different from mixing on a palette like oil or watercolor. Mixing on paper, if the sun light falls on 10 different spots in the painting, for each of the spot I have to mix to create the same color, and it's ok with bigger spots, but when the spots are tiny, mixing becomes nearly impossible. Also, mixing on paper is very difficult for atmospheric affect. Say there's a red house on the mountain that's nearest to us, and one on the mountain at the middle distance, and one farthest. The 3 houses, though all red, will all appear different depending on the atmosphere. The farther it is away from the viewer, the bluer, or grayer, and lighter it can appear. It is quite easy with oil, on the palette, you just add a little neutral gray to desaturated it, add a little blue or purple to tint it, add a little white to lighten it, and then you dab it on the canvas, and it's a perfect colour and any size you want. With pastel, unless I have the exact desaturated bluish red, I don't know where to begin to mix a perfect colour on such a small area. Maybe working big is the solution, or buying more colours, but coming from an oil background, where I could paint most things with 5-6 tubes of paint, it is extremely frustrating.

2013_06_11 - 6.5" x 8.5"

This one below was done a week later. Instead of mixing, I layers another colour on top. This approach is more direct, but so far has only worked for me in a few cases.

2013_06_18 - 6.5" x 8.5"

I am still painting with pastel, because it's easy to set up and clean, and it doesn't make an oily aftermath like oil; I could use it while at work. I do want to learn more about pastels, as it helped me pick up painting again.

The view for the 2nd one: