For future reference this worked:
Set pbAllowHtml to True // in cWebTreeView object

Move "The tree item description" to sDisplayName
If (bOk) Move ('<span style="color:green;"> ' + sDisplayName + '</span>') to sDisplayName
If (bHasErrors) Move ('<span style="color:red;"> ' + sDisplayName + '</span>') to sDisplayName

Send AddChildTreeItem (&aItems) sId sParentId sDisplayName sValue True True bExpanded sToolTip "" ""

Again, thanks Samuel!