ther is a method to set foreground and background of the whole tree, but tis is not what I want. I want to use different colors on individual items in the tree. Is this possible

Not by default in the GUI class library. Win32 Tree View custom controls support this via the custom draw functionality. See:-

msdn.microsoft.com/.../ff919569(v=vs.85).aspx

You would need to implement this yourself by subclassing the GUI Class lib TreeViewControl and  treeViewItem and handling the NM_CUSTOMDRAW message.

Regards

David