What are the key learning points of this activity?

The key learning points include:

Indicators do not necessarily change colour at pH7.

An observed colour may be due to a mix of colours (a bit like mixing paint).

Commercial universal indicator from a bottle is a mixture of single indicators.

This activity was originally written as a challenging investigation for the 11-14 age group. How could it be used with more senior classes?

You could consider other areas of content that could be drawn out and discussed, for example, buffers. There are questions to discuss, such as, how do we reliably keep this solution at pH3 when we know that indicators are weak acids? Then you could link into your introduction to buffer solutions, or, revisit and reinforce buffers if you have covered this area already.