Why?

I'm not sure if this is true but I think the titles that you already opened once before appear grey whereas ones that you've never opened are yellow

Shoot nevermind, I just realised the yellow ones dont change regardless

yellow titles mean a chapter was updated within the last 7 days, grey ones means it hasn't updated.

I think, featured manga shows which manga/manhwa was read by most people. And yellow ones recently gotten updated and grey one haven't update for one week or more.

Yup