If you select grayscale in the print properties, the images output will only be the red channel of the images. This can lead to odd results - it should use a perceptual mix of the three channels.
Created attachment 130935 [details] demonstration image - will print quite different to visual appearance
Comment on attachment 130936 [details] [diff] [review] use perceptual color mix of image channels r=pavlov
Created attachment 130963 [details] [diff] [review] make/use macros, fix setcolor as well