Colours: learn to love your palette
Oil painting means spending a minimum of 50% of the time working on your palette, never rush yourself!
Your Basic Palette: Titanium White; Cadmium Yellow Pale; Permanent Alizarin Crimson; French Ultramarine; Burnt Umber
For non-natural/unlikely colours use also: Cadmium Orange, Pthalo Blue and Pthalo Green
What is an unlikely colour? Something that you see extremely rarely in nature, like the orange in this rose:
Or the phtalo green of this wave:
Always obtain a pile of colours on your palette of a nice buttery consistency by adding your medium to the initial blobs of colours before starting colour mixing and painting.
You need to do this so that your canvas will have the same ratio of pigment and medium throughout, if you add medium as you go what will happen is that some parts will be "thinner" than others, creating problems when it dries and making it more difficult to gauge the second layer's medium.
Also, it's much more fun to go straight in and mix colours that are already prepared in your palette!
For this I follow to the letter the advices learnt from Mark Carder, which boil down to:
Black = Brown and Blue ○ For a warm black increase the brown, for a cool black increase the blue
Green = Yellow and Blue. For a very natural green mix yellow and black
Orange = Yellow and Red. For a warm darker orange mix yellow and brown
Purple = Blue and Red
To lighten up a colour: use Yellow or White or both
To darken a colour: use Blue or Brown or both
Be careful with the white! Once added in, you cannot take it out!
Mark's colour wheel is my total reference point: