Your capture of the test png shows that coreelec does not output RGB images even close to bit-perfectly. Out of the possible range of 0-255, the R,G, and B channels have maximum errors of (25, 24, 23) (14, 32, 8) and median errors of (9, 3, 7)(8, 13, 1) - essentially almost every byte is output inaccurately - and often significantly so. Given the large value of these errors I doubt it would be due to any sort of unwanted colorspace conversion and would guess it is an issue with coreelec/kodi itself.
edit: fixed error value now that captured data is being imported correctly