Hyperpixel4 - made HDMI screen go to rainbow colours

Hello - just plugged in the hyperpixel4 to my Raspberry PI Zero. The desktop now appears on the TFT but the HDMI display just shows a rainbow square box. I assumed that this was normal but after powering down and unplugging the Hyerpixel4 and then rebooting, the HDMI display remains a a rainbow. I assume some setting has changed but am not sure what or how to restore it.

Should I be able to keep the original desktop on the HDMI and to use the Hyperpixel display for a GUI for the RPI program that I am writing? So, keep developing using Geany on the big screen and test controls on the small? Can’t see any examples or how-to stuff after the easy installation one-liner.