Your developer may have become alkaline. Take care to use it with a pH of 6 or
less by adding citric acid to it. I take the test strips to control my fixer's
silver content and pH to check this. I need about 15-20 grams for two litres of
33% potassium oxalate. The brownish/yellowish color is a permanent iron pigment
formed in the paper fibers in alkaline environment.
Klaus Pollmeier