The light grey colour is due to the fact that the window in focus is the search panel. If you find something and then click on the source window, the highlight will change to the system highlight colour. This can be changed using the system preferences. The system default may be some form of grey too.
Here is the System Preference panel where the highlight colour can be changed:
If you choose 'Other' you will get a colour palette and you can choose any colour you like.
To see the difference in the focus, here are two images. The first, is when focus is in the Search Panel:
Now when I select the source window, the highlight changes to my selected colour (in this case bright yellow).
But the behaviour of highlights in windows that are not in focus is not changeable, I don't think.