That happens a lot with acrylic paints @CheerBear I add in Impasto (to create texture) and that often keeps some of the brightness too. You could add a touch of white to each colour as it will still dry darker and give a bit more volume to the colour you are using. Painting on black will absorb some of the colour too. Unfortunately a lot of acrylics dry much darker/duller though so it is a bit of trial and error. 

@Faith-and-Hope might have some better ideas so tagging her here too.