I'm implementing a device that is using MQTT to control some multi coloured LED lights via a microprocessor. When I change the light colour from the "Test Color" button on the Devices screen all is well and the correct 6 character hex code is sent to the MQTT queue. However, when I do the same from Alexa I get a spurious code, usually x"FDFDFF" but sometimes a slight variant.

Here's the MQTT string from the device:

{"clientId":"hab001","topic":"home/colour/test","message":${color.rgbx},"qos":"1","retain":"false"}