Older versions of Office programmes presented a choice of bright colours in drop down boxes, such as Track Changes. Current versions offer very subdued, muddy colours which don't stand out anywhere near as well as the bright ones. Is there a way to change the default colours from shades of mud to nice bright primary colours?

To some extent, colors offered depend on document themes.
I don't know that this extends to Track Changes, though.

Which older version?

This is a screenshot from my Word 2010.
00 deleteme 1.png

How to attach a screenshot or file in this forum.

I had Word 2010 with colours as in your screenshot. Unfortunately when our office jumped into Word 2016 we were left with these colours:

Attached Images

Capture.PNG (23.1 KB, 14 views)

I was relying on your login as Office 2010.
I suspect the color change came with the change to the text editing engine in Word with Office 2013. I know of no way to change these. Someone else may.