Fill in the blanks:
$(1)$ ......... acid is added to perform titrations with $KMnO_4$ and $K_2Cr_2O_7$.
$(2)$ Crystals of potassium dichromate are of ......... color,while crystals of potassium chromate are of ......... color.
$(3)$ The color of $MnO_4^{2-}$ is ......... and the color of $MnO_4^-$ is ......... .

(N/A) $(1)$ Dilute $H_2SO_4$ is used because it does not act as an oxidizing agent itself.
$(2)$ Crystals of potassium dichromate $(K_2Cr_2O_7)$ are orange-red,while crystals of potassium chromate $(K_2CrO_4)$ are yellow.
$(3)$ The color of the manganate ion $(MnO_4^{2-})$ is green,and the color of the permanganate ion $(MnO_4^-)$ is deep purple.