well I thought this was the answer:
Quote: "But still the point remains., only use "Pure Black" for Transparency / Alpha... use an Off-Black for Black as a Colour."
however I have run into this issue again (different project).
The results are rather unpredictable.
I have tried with both gif and png files with the same results.
Images with non transparent pixels sometimes do not show up depending on the background color of the sprite.
As, I say I cannot see a pattern of behavior yet it seems that if the background is close to 'black' then it influences both transparent and non-transparent pixels in the image
I also found that with a black background, pixels must be at least 15 units away from 0,0,0 to regarded as not transparent, not just one. In other words the 'almost' black is visibly not black.
Here is a file that has yellow and green areas:
No matter what background this is placed on, I believe that the area around the signal head should be transparent and the signal head should always be 'very black' and the green and yellow lights should always be pure colours.