it is more usual for acrylic colours to dull down a little. It really depends on what you use. I use Atelier interactive paints which have many other splendid qualities. I highly recommend hitching up with the Michael Wilcox School of Colour, as revelations occur regarding choice of pigments, and colour mixing skills are enhanced from 6 tubes or a max of 12 inc white [no black].
Dullness can also be addressed. Until recently I hesitated to varnish my work - then I used Chroma Solvent Finishing Varnish [Satin]. This gets rid of the "plastic" look of acrylic work, and deeply enhances the colours - particularly the darks.. In fact - a finished painting thus treated becomes an entirely different piece of work, and I remain thrilled with the end results.