I am having a lot of trouble mixing colour. Could you help please?

Use ultramarine blue with both lemon yellow and yellow ochre to get a range of colours. Mix in titanium white to get the lights. Underpaint dark purple with ultramarine and either vermillion or crimson red depending on whether the shade is cool or warm. This will give you your dark shade in the tree, then you can overpaint with a mix of greens, adding touches of lemon yellow on the light side of the tree. If the tree is in the background be sure to dull down your colours by using yellow ochre instead of the brighter lemon yellow.