Taking the colour that you have in mind to be a light, dull blue-violet (the normal description), I would suggest that you mix a violet-red such as *Quinacridone Violet with Ultramarine Blue.
*Crimson Alizarin could be used but it will fade.
Mix until you have a violet which definitely leans towards blue. Then, to make this colour both lighter and duller, add white paint. It will not be sufficient simply to apply the mix thinly and allow the background to lighten it. You will need to add the white in paint form in order to make the colour duller. If you have a copy of our "Colour Mixing Swatch Book" go to page 22 - a range of 'violets' will be found in the bottom right hand corner. I hope that this answers your question.
