I have now tried to connect a digital camera with the same setup (with hdmi input), and the results are still the same (don’t have sample pictures unfortunately). It seems that the HDMI input method “washes” the colors of the picture or that the whitebalance seems to be off. Is it possible to manually choose the video input and the video type/colorspace/resolution etc. on the capture card (without have to depend on the picamera library)?
That may or may not help.