Please run the test: ControlsAutomatedTestSuite/javafx/commons/ControlsTest/opacityTest

It produces two screenshots:
1 - screenshot of initial state of control is taken and then 40% opacity is applied ("_img_opacity40" suffix)
2 - control opacity is set to 40% and then its screenshot is taken ("_control_opacity40" suffix)
And the diff between two images shows that there is slight difference between them.

In the output you will see the test results directory where two screenshots and their diff will be placed