CrO42- exists in equlibrium with Cr2O72- in an aqueous solution.
2 CrO42− + 2 H3O+ ⇌ Cr2O72− + 3 H2O
By decreasing the pH, you shift the equilibrium to the right, Cr2O72− is orange in color. So the precipitate should become more orange. You can find the percent decrease in yellow ppt by using the Keq and Ksp to find the percent increase in Cr2O72− . In this situation knowing the pH before and after the addition of the acetic acid is the most important consideration. The change in the amount of Cr2O72− may be too negligible to detect a change outside of a photodetector.