I usually start Channel Mixer conversions at: Red 20%, Green 60%, Blue 10%. Images vary a great deal; that setting doesn't look quite right here though better than after desaturating. This image has three noticeable features: the skin tones of the painter, the relative brightness of the various blocks of colour on the batik and the brightness of the yellow wall to the right. My preferred solution is: Red 15%, Green 75% and Blue 10%. I would like a slightly higher overall brightness and a bit more contrast but would do that as a separate process.