The colours used for this piece are Permanent Yellow medium, Irgazine® Red DPP BO, Cobalt Blue Dark and Raw Sienna.
Kremer Pigment's Cobalt Blue Dark sort of the equivalent of Ultramarine of other watercolour manufacturers. This is an extremely granulating colour. I can even see sediments at the bottom of the glass of water that I wash my brushes with.
Interestingly, Raw Sienna and Cobalt Blue Dark mix to a neutral gray rather than a green.