Hi - if you can show some of your current pencil work it will help others in giving advice.
In drawing/painting with colour, of the three parameters of colour (hue, value, and chroma), the most important to concentrate on initially is (perhaps a little surprisingly) value (how light or dark the colour is, as compared to equivalent grays) - so if you can
shade effectively in "black & white" you are well on the way to being able to use colour effectively. (Hue is what non-artists mostly think of in association with colour - whether its red, yellow , blue, green.. etc; Chroma is how
intense the colour is).
Have a look at this page - all the coloured chips are the same hue (orange) - but they vary from top to bottom in value (see how they match the different values of grey), and they vary from left to right in chroma.
(click on the images to see them big)
Click to view attachmentOne needs to identify what value, chroma, and hue you are
aiming at, for a particular area of the painting, and then mix the appropriate colour. In oils, it's useful to premix different values (for example, a light, mid and dark version, or even a stepped series of different values) for different colours. Some people mix for
each patch of colour and then "tile" the painting with these patches - as long as each tile hits the correct target colour, the painting virtually "paints itself".
For example, here's one approach to a portrait palette in oils:
Click to view attachmentThe idea of this is that there are value strings in yellow (various mixes of white, yellow ochre, and burnt umber), red (various mixes of white and permanent alizarin crimson, here), and greys (mixes of white, black, and a touch of raw umber) as horizontal rows. Each vertical column here contains a grey, yellow, and red of approximately the same value. The idea is that you then mix between the red yellow and grey
from one column to hit the correct hue and chroma while maintaining the value:
Click to view attachmentDave