Is it correct not to perform ghusl after transparent and very light yellow discharge, with the approaching menstrual period?

Your action is correct, and ghusl (ritual bath) is not obligatory for you, as it is only required if you are absolutely certain that something necessitating ghusl has exited. As for the yellowish discharge: it is not considered menstruation as long as you see it outside of your usual menstrual period. Istinja' (washing the private parts) from it and wudu' (ablution) for prayer are sufficient.