Assign ID numbers according to groups

Button blue can add blue item, red button adding red button, etc. one button can be clicked more than once, hence there can be more than one blue or red item.

the table is as below

item     id      details    size
blue             cheap      small
blue             expensive  big
blue             cheap      small
red              cheap      small
red              ok         average

how can i assign the id for each of them so that the table is as below

item     id      details    size
blue      1      cheap      small
blue      2      expensive  big
blue      3      cheap      small
red       1      cheap      small
red       2      ok         average