How to do colour clustering on a PNG image with a transparent background?

Hi,

I have a sample PNG image the background of which has been removed. See below ( well it shows the white background here but please imagine the background is clear not white).

However when I display the image in Matlab using 'imshow' I get the image below (not surprisingly) on a black background.

My main goal is to do colour clustering on the image with the transparent background.

Now sadly for the image with the black background, the main 5 colours I extract out of the image are below with ( no surprisingly ) black being the most dominant colour. ( See below).

That is not I want.

I would like to do colour clustering on the image with the transparent background.

How can I achieve that?

Please advise.