I’ve just read this comment from an Affinity developer, maybe this is the key to solve the color issues while using Nik tools in Affinity Photo:
There is an update to this. The compatibility has been increased to the following (text copied from Affinity forums):
The color bug is fixed in those plugins as of Affinity Photo 1.7 beta 333:
Topaz Studio 1.14.4 Topaz Denoise AI 1.0.3 Topaz Sharpen AI 1.1.3 Nik Color Efex Pro 4 Nik Analog Efex Pro 2 Nik HDR Efex Pro 2
Other plugins that still display wrong colors when invoked from Affinity Photo 1.7.333 are:
Nik Dfine 2 Nik Viveza 2 Nik Sharpener Pro 3 Alien Skin Bokeh 2 Neat Image 8.3.7
Is there a big difference in color handling between the Efex plugins on the one side and Dfine, Viveza and Sharpener on the other?