After performing a windows re-install and using Tweak UI to change some unrelated settings, I found my desktop and program icons had all reduced in colour.
I was using a 16-bit display and the problem alleviated itself when I changed to a 32-bit colour depth. Going back to 16-bit though left the colours in their 256 colour glory.
Trying a variety of settings and deleting the IconCache.db didn’t help however the attached annoyances.org link, led me to a registry change.
HKEY_CURRENT_USERControl PanelDesktopWindowMetricsShell Icon BPP was currently set at 32.
On changing it down to 16 and rebooting, my machine went back to displaying icons in their 16-bit glory.