Is ghusl obligatory if there is wetness and a smell in the underwear, and a sky-blue color appears from the detergent residue, with 51% doubt that it is not the smell of semen?

You are not required to perform ghusl (ritual bath) merely upon doubting the emission of semen. Rather, it becomes obligatory only when you are absolutely certain of its emission. If you doubt whether what was discharged is semen or not, you have the option to consider it semen or not.