[Pharo-users] Is there a way to set form colours using a 'fuzzy' option?

I have an image with black text, and a white background. I would like to set the white pixels to transparent. I worked out how to do this, but of course it leaves me with a white halo around the text, where the border pixels are almost white, but not quite. In googling about this, I discovered that RMagick has the idea of replacing pixels which are almost any particular colour. I didn't find anything similar in the ColorForm class. Is there a way to do this within Pharo? Cheers Andy