I've been browsing Qt3 Designer sources trying to find something that could help, but Designer uses a completely different code and despite that I know it is possible to achieve what you want (Designer editor highlights matching brackets when cursor is on one of the brackets), it looks like it's not possible without much much work. I was unable to find anything that could help and I fear that even if I did, you wouldn't be able to use it in any easy way anyway. So either you need to:
1. think of something different,
2. do the colouring yourself by overriding some paint event, calculating the position of characters you want to colour and paint appropriate areas before the text is rendered,
3. switch to Qt4