My current problem is that I would like to have colorized 923 and 920 toolbar Pictures. However, setting the usual <Metadata><Colorize>true</Colorize></Metadata> won't do its job as the images still show the default color (aka the real contents of my replaced PNG file), not a colorized version. The question is:

How can I colorize toolbar icons (e.g. 410-457, 1441-1489)?

Well, some pictures cannot be colorized but I think there is a solution for that. You need to replace the default resource id (e.g. 1441 with 59000) of each button in the style files. But, I suggest you do it first for only one button to see if it works and then try it on the other buttons. Again, I am not sure if this will work, but it worked for some other pictures I wanted to be colorized. I hope I helped you