Hover over grey images to colored

Site URL: https://www.bluestonemanagement.com/

Hello!

I am trying to create a hover effect over the first six images (Project Management, Quantity Surveying, etc.)so that when hovered over, they go from greyscale to colored. I'm having trouble finding/selecting the correct class (because nothing is happening when I do anything to the classes I find). What is the custom CSS needed to achieve this goal?

Everything you need  to know for b/w to color or vice versa is here and the transition timer for the change as well.