In A3 you can enter this formula:

=A1

or better still:

=IF(A1="","",A1)

However, a function cannot change the format or colour of a cell - it
can only return a value.

One possibility you might consider, though, is to use conditional
formatting on cell A1 to give it the colour (eg if it is not blank,
then make background colour green). Then you could apply the same
condition to A3 so it will seem as if the colour has been transferred
automatically.

Hope this helps.

Pete