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 Visual preferences page, which can be accessed from the preferences page:

In the Visual preferences section, click on any price from the image or selection any type of price on the right menu will open the prices preferences section:

On this section, you will be able to select the colour for each price by using a color picker:

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