Grumbacher Thalo Blue
CYAN

All opposite colors mix to make a neutral dark.  This combination of primary and secondary oppositions or any two opposite colors naturally makes a perfect Neutral Black or Gray of any shade. This combination is seen mostly in the evening. Finding the neutral-dark balance between these two colors is tricky and hard to hold, but it's there when you want it.

I choose Grumbacher's Thalo Blue because they have the correct hue and they were first, it's very important what color a transparencies tints to. Thalo Blue Transparent as a tint will match the mass-tone tint of Schevenings Blue Light, Opaque.