[Cr(H20)]3+ is violet. another CrL6 complex is green. Can ligand L be CN-? Can it be Cl-? why?

Answer 1

First of all you have not typed (obviously) correctly the question.
[Cr(H2O)6]3+ is violet and thus you are comparing the colour of complexes of Cr+3 with different ligands but the same co-ordination number.

The stronger the field by the ligand, the bigger will be the energy difference in the d orbitals of Cr (bigger splitting). The order is
Cl-< H2O
The bigger the splitting, the higher energy radiation will be absorbed. This means higher frequency or lower wave-length.
The colour of the compound will be the complementary of what is absorbed.
Green and violet are complementary colours.

So [Cr(H2O)6]3+ which is violet absorbs green
CrL6 which is green absrobs violet
Green corresponds to lower energy than violet.
Therefore L should cause bigger splitting than H2O which means that L could be CN- but not Cl-

So your complex could be [Cr)CN)6]3-

Answer 2

I think CrCl6 is green, but check with someone first.