my external display is blue

iMac (Retina 4K, 21.5-inch, Late 2015)

I have connected a Samsung Monitor via HDMI to mini display

However, my external display, though functional, is blue and changing the "color" or color profiles don't make any changes.

The last time I logged off my computer, the display had the same background as my built in (not mirrored)

HELP