Colour new content in Output

There's no built in way to do that. If you have a developer they might be able to write a script that you can run -  believe there's a field in RH2020 that allows you to run a script automatically after generating. But I can't imagine any way of marking specific text within a file automatically, only all content within the file. (I suppose if you have source control maybe there's something that could be done with file comparison tools, but I think it would be awfully complicated.)

Another option is to use condition tags. If you don't exclude the condition using a condition expression in the output, the content and applied condition tag remains in the output and you can apply a style based on the condition tag. You can try the following in a test project. In a topic, apply the default condition tag "Platform: Online" to a paragraph. Switch to code view and copy the condition tag data-condition="Platform:Online". Open your stylesheet and switch to code view. At the end of the file paste in the condition tag you copied and add square brackets around it [data-condition="Platform:Online"]. Add the curly brackets and add your style. 

[data-condition="Platform:Online"] {
    background-color: pink;
}

Using the above example, any text tagged as 'Online' will have a pink background colour in the output.

The way this would work is as you are working on a new release, you'd mark your changes with a condition tag (say 'NextRelease'). When you generate, everything with this tag will get the styling you specify. When you start the next release, you delete the condition tag and start again.

I just discovered that in RH2020 deleting the condition tag removes all references from topics - I haven't checked if it also removes the tag from other places like index, toc, etc - but even if it doesn't remove it from everywhere, it's not too hard to go through and manually deselect the tag instead of deleting it (I've used a variation of this method to produce a draft output for review in Classic).