Hello, prasum!

This one is very easy.

Exactly where is your difficulty?

Perhaps the idea of six sizes confused you.

This was only to show that the six red balls are distinguishable.

. . They could have been simply numbered: .

There are 6 choices for a red ball.

There are 6 choices for a yellow ball.

There are 6 choices for a blue ball.

There are 6 choices for a green ball.

Therefore, there are: . ways.