Hi @WillAsh,
Welcome to the community.
There is a way, sort of, to change this with Xperia Keyboard. Go to Settings > Language & Input > Xperia Keyboard > Skins. You can change it to a different skin and the word suggestion will be the colour of the theme.
If you only want to change the word suggestion highlight colour, it's not possible with the default keyboard. Android is completely customisable though, so you can find another keyboard from the Google Play Store that may have this functionality.
