Lolcat clone in x64 assembly

lolcat.asm

section .data
    char_buffer db 0

section .text
    global _start

_start:
    mov r12, 0
    .loop:
    call read_char
    mov rdi, rax
    cmp rdi, 0
    je .end
    call put_char
    mov rdi, r12
    call set_color
    inc r12
    jmp .loop
    .end:
        call exit

read_char:
    mov rax, 0
    mov rdi, 0
    mov rsi, char_buffer
    mov rdx, 1
    syscall
    cmp eax, 0
    jg .success
    mov rax, 0
    ret
    .success:
        mov rax, [char_buffer]
        ret

put_char:
    mov [char_buffer], rdi
    mov rax, 1
    mov rdi, 1
    mov rsi, char_buffer
    mov rdx, 1
    syscall
    ret

set_color:
    push rdi
    mov rdi, 27 ;escape
    call put_char
    mov rdi, '['
    call put_char
    mov rdi, '3'
    call put_char
    pop rdi
    mov rax, rdi
    mov rdi, 6
    div rdi
    mov rdi, '0'
    inc rdx
    add rdi, rdx
    call put_char
    mov rdi, ';'
    call put_char
    mov rdi, '1'
    call put_char
    mov rdi, 'm'
    call put_char
    ret

exit:
    mov rax, 60
    syscall