Change Colours In Eclipse Diff

Inside of the properties you can change all the colors and fonts. You are looking for

General -> Appearance -> Colors and Fonts -> Text Compare

I find it hard to see the diffs in the eclipse built in diff as everything is a shade of grey

Not anymore with Eclipse 2019-09. You don't have to change the color of the text itself.

In simple cases such as a 2-way comparison or a 3-way comparison with no merges and conflicts, the Compare Viewer now shows different colors, depending on whether text has been added, removed or modified.
The default colors are green, red and black respectively.

In Dark mode, the default "black" for edits is hard to see. You may want to change it (for example, to orange)