Every ink company has a different shade of Royal blue stock ink. So depending on which Royal you are starting with, but typically you need to add a little green then small touches of black until you get it dark enough. Keep a sample print when you do this so you can match to it again if you get a reorder. Just staple a swatch to the work order and file it away. The only colors you cannt mix are the primary colors...Bright Red, Lemon Yellow, Bright Blue and Purple (Violet)....and black and white of course.