Opposite colors mix to make a neutral dark.  This combination of primary and secondary oppositions naturally makes a perfect Black or Gray of any shade. 
This combination is seen mostly in the evening. 
Finding a 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, it's very important what color a transparencies tints to. Thalo Blue Transparent as a tint should match Schvenagen Blue Opaque.