Areas with no hair vs areas that have hair density will make the colour variations appear more dense.
To counteract this you can choose to prepare two blends – the first as the colour you want for the hair and the second with a small ratio inclusion of a deeper colour in the range. Apply your first chosen colour to the entire brow for your first layer. For your second layer apply your second (deeper colour) blend to the areas with skin only and allow to dry. Apply a second coat of your first colour again for your third and final layer.