@Michaelvkempen First set a variable in each one of the buttons,
In the OnSelect of the OK button,
UpdateContext({rectangleColour: Color.Green})
In the OnSelect of the N.V.T. button,
UpdateContext({rectangleColour: Color.Gray})
In the OnSelect of the NOK button,
UpdateContext({rectangleColour: Color.Red})
Finally, set the Fill property of the rectangle to,
rectangleColour
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