Insert values from 0 to 15 on keyboard fasas x86 assembly

Good. I have to do a program in assembly that le a keyboard number which is to choose a color in video mode. From 0 to 9 I know it's like this:

mov ah, 40h                  ;Pedir um valor
mov bx, 1
mov cx, 44
mov dx, pergunta
int 21h


mov ah, 3Fh                     ;Introduzir valor
mov bx, 0
mov cx, 1
mov dx, valor
int 21h

sub [valor],48

Now I need help to be 0 to 15