The Chromate - Dichromate Equilibrium

What happens if we add some hydrochloric acid?

For simplicity, the symbol H+ is being used for the acid proton, rather than the Brönsted-Lowry hydronium ion H3O+.  All ions in these equations are aqueous, but the (aq) symbol has been omitted for brevity.

There are always a few H+ ions in water.  Hydrochloric acid is a concentrated source of H+ ions.  If there was no chemical reaction then adding HCl should increase the amount of H+.  The original  H+  in the water would increase to become H+  However, we wouldn't see any color change since H+ is colorless.

However, there is going to be a chemical reaction, since CrO42- reacts with H+.  From le Châtelier's principle, we know the reaction will try to remove some of the H+  we have added.  Some of the added H+ reacts with the CrO42-.  This removes some of both the H+ and CrO42-, and make more Cr2O72-.
 
For the chromate solution (remember that the original solution is an equilibrium, so it contains mostly yellow chromate, but a bit of orange dichromate):  

add some H+ (aq) 
and it becomes		 a much more orange solution
Original condition 2 CrO42-  + 2 H+   Cr2O72-  +  H2O (l)
Immediately after some H+ is added
(since reactions take time, we can
imagine what the system is like before it
has time to react)		 2 CrO42-  + 2 H+  Cr2O72- + H2O(l)
New equilibrium is established,
removing some of the added H+		 2 CrO42- + 2 H+  Cr2O72- + H2O(l)

Notice, that the H+(aq) is less than it would be if there was no reaction, but greater than it was at the beginning.  There has been a big increase in the amount of orange dichromate.

For the dichromate solution (remember that the original solution is also an equilibrium, so it contains mostly orange dichromate, but a bit of yellow chromate):

add some H+ (aq) 
and it becomes		 a slightly more orange solution, and probably not noticeable
Original condition 2 CrO42- + 2 H+   Cr2O72- + H2O (l)
Immediately after some H+ is added
(since reactions take time, we can
imagine what the system is like before it
has time to react)		 2 CrO42- + 2 H+  Cr2O72- + H2O (l)
New equilibrium is established,
removing some of the added H+		 2 CrO42- + 2 H+   Cr2O72- + H2O(l)

Notice, that the H+ is less than it would be if there was no reaction, but greater than it was at the beginning. There has been an increase in the dichromate ion concentration, but since there was a lot to begin with, the change isn't very noticeable.