I saw a recent YouTube video of someone using a Lightbox to take macro images. They looked great but looked better when they inverted the white background to become black. The method they used to do this in photoshop was relatively straight forward. They changed the colour gamut to LAB profile for the image they were editing in PS. The "L" channel holds the black and white details and there is an option in PS to invert this channel. I am trying to find a way to replicate this process with Affinity Photo and cannot see to find a way to invert a channel.
Anyone have any ideas?