Help with Guage Colour gradient Fill

[Henrybukowski]

Using a template I found on the internet I've created a thermometer guage linked to a formula in another sheet. The 'mercury' itself is a shape plotted with a fill based on the value of a cell.

I've formatted the mercury level as a gradient fill using a red to green traffic light spectrum.
The problem is that the colour gradient adapts to the space it fills, so that you get the full colour spectrum even where the mercury level is low.
So my question is whether there's a way to have certain colours only appear after certain levels?

 

[Fluff]

Cross posted https://www.excelforum.com/excel-general/1226176-help-with-guage-colour-gradient-fill.html

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