Heatmap visualisation colours

[desc]

Hi

I am using a heatmap in one of my visualisations and I want to know what I'm doing wrong. I want to modify the standard colour palette;

I thought I could do this by editing the json file where the colours are expressed in HEX format. When I do this, republish and select the heatmap visualisation - the control no longer works. 

Is there something else I need to do to the json file in addition to adding the new colours I'm after?

Thanks