The light yellow color could be due to charring. Try adding the charcoal and filtering through a plug of celite(diatomaceous earth). Alternatively could could try shorter refluxing time. Hope this was helpful?

dear Vimal,

I just found out pure dop is actually yellow in color. Look it up in "Natural Product Isolation by Sarker and Gray. Cheers!

Thank you for the answer, But I have already tried charcoal and shorter refluxing time is not possible in order to complete the reaction. I have also tried nitrogen inert atmosphere.