May I know how to find out the yellow side?

You mean how to work out the number of small cubes with all yellow sides?

Only the small cubes that are on the outer surfaces of the original block will get any green paint. So imagine removing off the outermost layer of every face of the 4 x 4 x 4 block. The remainder is a smaller 2 x 2 x 2 block, making a total of 8 small cubes with all yellow sides.

Understand now. Thank you very much for your explanation