What sorts of household substances are good to test?

There is a huge variety that you can choose from. Experiment! Popular examples of acids are vinegar, lemon juice, cola, fizzy drinks and tomato sauce, while toothpaste, washing powder, baking powder, soaps, detergents and indigestion remedies are alkalis.

What colour is the indicator in acid and alkali?

The range of colours looks like this:

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Natural indicator versus laboratory indicator?

The naturally occurring indicator is cheap and easily extracted from widely available materials. It is good enough to determine the groupings of acids, alkalis and neutral substances. A laboratory indicator, such as Universal Indicator, has a more intense colour so only a very small amount is needed. It can also show the full range of pH values, differentiating which substances are more acidic than others, and similarly determining the rank order of alkalis.

How long will the indicator keep?

It will keep for a week if it is stored in a sealed container in the fridge.