expressions - How could I express the idea that one person is learning the habits of other person?

"picking up their habits"

Example usage:

I am picking up my sister's habit of listening to the Beatles.
My second child is picking up all the bad habits of my first child.
I'm picking up the habits of my roommate.

If you particularly want to use "colour," it would be something like this:

Steve is really colouring the behaviour of his partner.

...but it is not an idiom like "picking up habits..." and I would not recommend using that.

Incidentally: there is a somewhat humorous idiom "Well colour me happy!" which you may be thinking of, but it is not really related to habits and habit forming.