In developing a plugin I met a problem with the toolbar icon. I followed the instructions in the help file and for 256-color icons I decided to make RGB(1, 1, 1) a transparent color. So, I designed an icon with 4 pixels in corners with this color (to make it look a bit rounded), and in the code I wrote:

However, when I try the plugin in 256-color mode, its toolbar icon becomes garbled. Here is the screenshot of how the icon looks as itself and when shown by EmEditor:



EE 9.07; OS Vista Business SP2 x32 working in true-color mode.


<EDITED>
I performed some more experiments and found the following:
1. If I return CLR_NONE instead of RGB(...), the color with index 0 from the image's index table is used for transparency.
2. The problem seems to lie in fact that my transparent color has index 0. When I move it further in the index table, the problem seems to be gone.

The questions are: what are the "official" recommendations on creating transparent images, and why is all this not described in the help?