Getting proportions easier

This is a small 8x10"portrait of Sarah I did from a photograph. I was using my computer monitor to view the photo, zooming in to get the detail but I was really having trouble getting it right. Then I printed out a picture of her face the exact size as I was painting it. I placed it next to the painting and it was much easier to get the proportions correct.
In other words, having the photo the same size as my painting kept me from having to interpret and recalculate in my head the proportions and sizes on my painting. I could visually reproduce it much easier.

Limited Palette

I use whatever colors I feel like at the time. I don't limit myself just for the sake of having a limited palette. However, most of the time I only use the primary colors to create a painting...

Cerulean blue and ultramarine blue and , - Indian yellow and cadmium yellow light, - Alizarin crimson and Napthol red (or cad red). This gives me both a warm and cool color for each primary color.

It's surprising how many variations including greens I can get from these few colors. I occasionally use black to gray my colors a bit. This also gives me one set of primary colors that are transparent and the other opaque. I start by using the transparent colors and then work into the opaque colors.

Mixing greens

During the summer months everything is so green. It often makes painting landscapes difficult because of the lack of variations with the color. When needing more variations with greens, it's often tempting to buy more green tube colors and use them get different hues. Most of the time adding more greens to the palette doesn't work very well. I find it best to keep my palette simple and mix most of my greens with blues and yellows often adding a touch of red to the mix. Adding more colors to the palette will more often than not simply make the painting less harmonious.
I usually start with a wash of of burnt sienna on the board, let it dry and then work my greens over that. It lets some warmth come through and dampens the harshness of the greens. Remember that all the greens in a scene like this will reflect and influence just about everything else like the rocks. A touch of green in all these other elements will give the painting unity.