When you make regular buttercream, it generally takes on the colour of the butter and has a creamy yellow colour to it. Now that's great if you're not bothered about the colour. However, if you'd like to get yours into a whiter shade, then all you need to do is the following.
Simply add tiny amounts of purple concentrated food dye with a toothpick, yes, that's right, I did say purple. Do it in stages and mix it in. You will see the 'yellow tinge' start to disappear and the white transform...