reaction 既observation 一問 [緊急！！！！]

I hope you understand english.
As you said, dichromate reacts with HCl. In acidic solution, the species is dichromate (orange, Cr2O7 2-). In alkaline solution, the species is chromate (yellow, CrO4 2-).
There is no observable change as the pH is low, dichromate will still be dichromate. That's so simple. It would not be chromate ion.
Moreover, only Cr3+ is green. Here, oxidation to give Cl2 and reduction to give Cr3+ is difficult as the oxidizing power of dichromate is not very high. Moreover, the reaction rate is too slow.