The color of the finished sulfuric acid turns yellow-green

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The process of producing acid from pyrite by blending it with alkylated waste acid involves injecting a small amount of alkylated waste sulfuric acid into the boiling layer, where the temperature is around 920 degrees. The waste sulfuric acid should be fully decomposed at this temperature. However, after increasing the amount of waste acid injected recently, the color of the resulting acid turned yellow-green; this change was most noticeable in the acid from the first absorption cycle, while the situation was slightly better for the acid from drying and the second absorption cycle. I analyze that it might be because the waste acid contains a small amount of chlorosulfonic acid; the HCl gas generated during cracking is not easily absorbed by the purification system and thus ends up in the absorption system. So the question is: why is it the absorption system that suffers? Could it be because the drying system operates under negative pressure, which hinders the absorption of HCl? Also, if it is due to the waste acid, why did this phenomenon not occur when the amount of waste acid mixed in was small?