Landscape Painting

The biggest cause of flat colouring isn't the colours but the mixing. The reason is that the better the colour is mixed the less the viewer's eye has to work. Impressionism works so well because it makes you work.
Brush mixing is the worst because it is the most effective. If you must mix beforehand use a palette knife. Best is to choose the right range of colours and let them merge on the canvas.
Also, if you look closely at skin coloration it varies much more than you may realise, especially in the shadows.
Take a good look at the likes of Monet, Seurat and later more extreme painters like the German Expressionists and Schiele. Those styles were developed for good reason.
And never stop trying things out, Art is path that has no end. It must be an adventure or you go flat.