Many modern papers contain incorporated developers. They are easily tested for activity and level by fogging the paper and adding a drop of strong alkali. Low levels or weak developers in the paper will allow it to brown or darken quickly, and high levels cause immediate heavy blackening. No incorporated developer leaves the paper a normal white or sometimes light buff colored.
These incorporated developers were not just present in some papers, they were used in general practice to give faster development times or to cause certain development effects.