How to use "Custom Colors" in the "Color" Drop zone to set a specific color to a specific value in a grid.
1) Create a new formula in the formulations app
2) Create an "IF" statement based on a measure using the "RGB" function and save it.
Use an RGB Color picker such as the one here to get the correct values for the colors you need.
3) In your report, find and drag and drop your custom formula into the Color Drop Zone>Background(or foreground)>Custom Colors
4) As can be seen below the grid foreground is now coloured according to color and value.