Hold your painting up so that visually it is adjacent to what you are painting. Make sure the light on the painting and the subject are consistent. Now you can mix a colour , place a little patch on the edge of your canvas, and directly compare it with the colour in the scene you are trying to match.
Hold your thumb over a patch of colour so its shadow falls on your patch. The tone and colour is exactly what is reqiured for the next tone down in painting your object.
Colour Matching. Choose the closest colour youve got and mix it to the correct tone. Select the colour immediately beside on the colour wheel on the side you want to shift. Mix this to the correct tone. Now mix the colour. Some adjustment for chroma may be required.