The best way to do this is to create a custom control and choose an image to replace the default look of the boolean button. To create a custom image on a button control follow these steps:
1. Create two image files with the desired look you want for the True/False positions of the button.
2. Add a boolean button to the front panel in LabVIEW. (something like an OK Button or Push Button)
3. Right click on the button and select
Advanced»Customize... 
4. On the new window select
Edit»Import Picture to Clipboard... to select the first image you want to place on the button.
5. Now select the True/False state you want the image associated with by right clicking on the button and going to
Import Picture from Clipboard»False. This will place the selected image for the False state of the button.
6. If you want to select a different image for the True case, then start by selecting a picture as explained in Step 4 and select the True case in Step 5.
7. Save the modified button as a .ctl file.
8. To insert the modified button on your main VI you can go to the Controls Palette, browse to Select a Control... and select the .ctl you created.
For more information on editing controls, please see the
LabVIEW 2018 Help: LabVIEW Custom Controls, Indicators, and Type Definitions .