E. Coli metabolization of paracetamol
Sorry that my answer is not a real answer, but this would be a good starting point for some more science. 

Several of the questions you might ask and could answer (with some more experiments): 


Does this happen in plates without E.coli?
You have a contamination on the right side, perhaps also on the black part. Could this be the culprit? Only half the plate is black, the rest is (still?) red. 
You used a paracetamol tablet with red dye and a lot of sweeteners, does the same thing happen if you use one without the red dye? A dye would be a likely source, because it already contains an absorbing moiety. It only needs to be slightly modified to absorb different wavelengths.
What kind of agar are you using exactly? There are many kinds, often selecting for certain types of micro-organisms, with indicators for pH or other colored markers to detect and identify strains. 


For your metabolic question you can take a look at https://ecocyc.org/ where you can find a database with all known metabolic pathways for E.coli K12. Unfortunately I couldn't find anything for paracetamol (or synonyms) but you could try to see if it produces a colored product for any of the additives.