Filter plugin - Pixels become grey when alpha set below 50 (ish)?

Hi,

My filter plugin modifies the alpha channel values of various pixels and I noticed that when an 8 bpc pixel's alpha is set below 50 (or so) that the pixel ends up turning grey, as if the R,G,B values were all set to the same value. Is this Photoshop's behaviour when painting/compositing pixels that are very transparent or am I confused? :-)

Thanks,
Neil