On an iMac, I tried Graphic Converter, which has several settings at Picture/Invert.
The Normal setting gave the best result, although still with a blue cast overall. So using Effect/White correction, and focussing on one of the flowers on the bush in the centre left, the clouds were clearly separated as white against the blue sky, the grass and shrubs on the bank gained a passable green, but the road, pavement wall, subject's face, and castle tower still retained a bluish tinge., resembling old hand coloured monochrome photos.
Further attempts at adjustment kept resulting in a number of small, features, which were previously hardly visible (such as handrails (?) on sloping paths running diagonally through the shrubbery across the bank, and minor facial features on the subject) becoming highlighted by a limited number of pixels in those locations which changed to bright red.
--
Cyril
