Insert values from 0 to 15 on the keyboard in Assembly fasm x86

Question

Good. I have to do a program on Assembly that reads a number from the keyboard that is to choose a color in video mode. From 0 to 9 I know that’s so:

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