Clear PNG changing colour of fill

[grd]

Hi

I occasionally insert PNGs into my worksheets either as logos or my signature. They have been configured to have a clear background and normally they work fine either on a clear background or over text, etc. However, today I have added a PNG over an area of fill and whilst all seems fine on screen and in print preview when I print the document the shade behind the PNG has darkened.

I've tried changing fill, saturation, etc in Format Picture to no avail.

Does anyone have any ideas please?

Thanks