How do you stop your colours from fading so quickly?

I find all my work fades so quickly. When its wet (i paint wet on dry) its great - when it dries its awful. Muddy and dull.

I have mostly Winsor and Newton Cotman tubes (lotsvof 'hues') but have very few professional tubes (because they are very $$).

Any idea what I could do to keep the colours vibrant?