Now I would say Turner was very positively a colourist - the colours are used to enhance the subject and create luminous paintings that glow with an inner light - this is done by his use of colour. In his abstracted works there is only minimal drawing as such and everything else dissolves in light and colour.
I would think someone who underpaints in tone and then adds colour usually is simply a tonal painter. A colourist painter would work in colour from the beginning and wouldn't want monochrome colour shining through, This is the way that a lot of old masters worked as you all know. The 'colourists' of the post impressionists, Scottish colourists etc broke away from this tradition because they wanted to use colour in a more exciting way.
Georgia O'Keefe's flower paintings, though colour
ful, are not colour
ist. A colourist looks at all the subtle changes of colour across a flower, the colour of the sky in the highlights if it's outside, the reflections of nearby colours in it and the mood and feel created by the colours, transparency vs opacity etc etc
Sally Strand's pastels are a good example of a colourist - paintings of sportsman in white shirts which are pure colour, but read as white.
These are purely colourist as
I feel it is defined - we'll continue to disagree
A colourist uses colour for what it
does, not just to make a brightly coloured image. A tonal painter paints shadows and folds primarily or solely in darker shades of the
same colour.