Which one of the following sulphides is yellow?

Assertion : Coloured cations can be identified by borax bead test.
Reason : Transparent bead $(NaBO_2 + B_2O_3)$ forms coloured bead with coloured cation.

Pick out the incorrect statement:

$A$ metal $M$ which is not affected by strong acids like conc. $HNO_3$,conc. $H_2SO_4$ and conc. solutions of alkalies like $NaOH$,$KOH$ forms $MCl_3$,which finds use for toning in photography. The metal $M$ is:

The color of acidified $K_2Cr_2O_7$ is not changed by:

Which compounds are formed when $Mn_2O_7$ and $CrO_3$ react with water,given that they are acidic in nature?