On the top of the phone there's a light that flashes blue when you get a text. Is there an app or setting that can change this so the colour is different for some senders who text you?
There is powerful and full-customizable GO SMS PRO which lets you set any LED color for sms incoming notifications based on sender. Also, it is a mighty alternative for stock sms app and has many useful functions and customizations.
After you installed it, run and tap menu, then Settings, and go to Advanced -> Individual Settings and add any contact to set individual notification (including LED options) for it.
Many custom ROMS, such as cm10, have built-in support for changing the LED color.