How to interpret Percent identity matrix created by Clustal Omega?
Gene-1   Gene-2    Gene-3
Gene-1  100.00    16.18     20.35
Gene-2  16.18    100.00     29.66
Gene-3  20.35     29.66    100.00 


Gene-1 and Gene-1 have 100% similarity (and all the other diagonal elements). Gene-1 and Gene-2 have 16.15% similarity, and so on. Therefore the matrix is symmetric M(i,j) = M(j,i). 

You can also show just the upper triangular part of this matrix and that would be sufficient.

        Gene-1   Gene-2    Gene-3
Gene-1  100.00    16.18     20.35
Gene-2           100.00     29.66
Gene-3                     100.00


The colours basically denote the type (molecular nature) of residue. Have a look at this and this.