2 bugs; I guess.

1)
When I try to create (all this via C++) a PANEL which has not a bitmap assigned, but is initialized with the respective rgb-values, I have a problem adding another button (which is a bitmap).

It seems as if only one color channel of the added button-bitmap is displayed

2)
When I set the panel flag to ^= INVISIBLE, the panel does not actually disappear.

Although invisible it still is a hindrance to access panels of a lower level.

You can find the description of panels in the manual.

You can't set "INVISIBLE" for a panel, and you can not add a button after a panel was created. All panel elements must be already defined within the panel definition. Panel colors affect the images of all panel elements.

http://manual.conitec.net/apanel.htm
http://manual.conitec.net/button.htm
http://manual.conitec.net/apanel-rgb.htm

Please check out the examples how to create a panel and define a button. If you then still have problems, please ask again on the forum and describe your problem in more detail, with a code example so that people can see what's wrong.