Lord Discordant mounted on A Helstalker Daemon Engine
This is fundamentally the same paint scheme as the Word Bearer Lord 'I made earlier'.
The base is painted with Green Stuff World Colorshift paint. This is an iridescent paint depending on fine structure to bounce varying wavelengths of light in different directions - in this case red-green.
The effect is of a warp energy field with different shades of colour depending on light source direction and angle of observation. It doesn't really come over in static photos.
Note the difference in colour from paint in shadow and direct light.