I have started using a white primer layer when printing on darker fabrics.
That being said, you will see that the quality and opacity of the color is decisive. The following pictures will show that in comparison the white is not particularly strong. Therefore I decided to add another layer of primer from a different supplier, which had a much stronger effect.
With exact placement of the stencils adding additional layers is no issue. Further, I have been asked whether I have seen any adverse effects due to multiple layers, which is not the case.