Ibuprofen Gel 5% with HEC

During our preparation of ibuprofen gel 5% (ibuprofen, hydroxyethylcellulose, Benzylaclohol, isopropanol, sodium hydroxide) we notice that the gel has a darker yellow color as otherwise. The view of the raw material HEC looked as yellowish/white as the previous fate. What can be the cause of this more yellothing colour?