How do I change the colour of the prices?

In Also Bought you can configure the regular price colour(prices that are not on sale), the sale price colour and the 'compare at' price colour from the app preferences page.

In the Product Price options section, click on the Advanced options link to display the colour options.

When you click on the Regular price colour, the Sale price colour or the Compare at price colour boxes, the colour picker will be displayed for you to select the colour of your preference for each price.

The colour picker allows you to manually enter the colour value in hexadecimal format.