ELISA troubleshooting
The full oxidation of TMB ( 3,5,3’,5’-tetramethylbenzidine) is 2-electron process.  The first step (one electron oxidation) gives the blue product, which is then further oxidized to a yellow product (2nd electron) that is stable in acid.  At high concentrations this might appear brown.
You might also not be fully quenching, leading to a mixture of blue and yellow products (should be more greenish though).  In this case add more stop solution.
TMB chemistry:
Josephy, P. David, Thomas Eling, and Ronald P. Mason. &quot;The horseradish peroxidase-catalyzed oxidation of 3, 5, 3', 5'-tetramethylbenzidine. Free radical and charge-transfer complex intermediates.&quot; Journal of Biological Chemistry 257.7 (1982): 3669-3675.