When graphics hardware multisamples pixels for scaling it does the 4 channels RGBA separately so even if Alpha=0 the RGB from transparent pixels impacts on edge colors.
Photoshop doesn't seem to allow setting of transparent color to anything but white so you need to either start working with 4 channel images or process the images once loaded with that readpixel/writepixel method I explained above.
Photoshop doesn't seem to allow setting of transparent color to anything but white so you need to either start working with 4 channel images or process the images once loaded with that readpixel/writepixel method I explained above.