Hi there Andrew,
The display should be showing it’s maximum 18-bits of colour as stated on the store page if it has been set up correctly and sent to you undamaged! Could you maybe post an image of what your display looks like?
Also, could you double check that all of the pins are firmly in place and have good solder joints, just to be on the safe side. :)
There was a similar issue not long ago here, have a go at the fix: HyperPixel Color problems
If that doesn’t solve your problem, drop an email to support@pimoroni.com and they may be able to help you (maybe mention this thread so they can see an image of your issue; they may have a fix).