Wool carpet become yellow after i apply brown out

Have you tried a acid rinse ? What was the ph of the brown out ?...Did you use a wool safe product while cleaning ?

From the picture it looks more like a dye transfer than a brown out

was the yellowing there before browning treatment was applied?if not then i would suggest the browning treatment has reacted with some sort of cleaning product the customer has used previously as browning treatments due to their acidic nature are generally safe on wool

is that a hard surface next to it?? eg vinyl hard to tell in pic.
If so Could it be a chemical reaction from floor cleaner used on hard surface split onto carpet during mopping.
??
you need to get the ph levels at and next to the coloring, see if they are the same then work through it.

There are several varieties of brown out or brown removers. Some are oxidizers or reducers while others are acid based.

The acid type browning removers are safer for wool. The oxidizing browning removers often cause yellowing. The reducers can once in a while.

I suggest you find out the specific chemistry of the browning remover you used, what are the key ingredients, then get a meter and take a pH reading of the affected and unaffected area of the carpet.

Sometimes the correction can be accomplished by getting back to the unaffected pH which should be just on the acid side of neutral for most wool.

If the brown remover was an oxidizer or reducer, you can try to rinse it away and then use an acid like Fab-Set or citric acid powder disolved in water or you could go to the opposite chemistry. (Of you used an oxidizer, apply a reducer, etc.)