Image histogram without ready function

Question

I'm new to MATLAB.I want to extract a histogram of an image

clc
close all
clear all
picture=imread('space.jpg');
subplot(2,3,1);imshow(picture);
subplot(2,3,2);imhist(picture(:,:,1));title('red         ');
subplot(2,3,3);imhist(picture(:,:,    2));title('green ');
subplot(2,3,4);imhist(picture(:,:,3));title('blue ');

subplot(2,3,5);imhist(picture(1,1,1));title('white '); subplot(2,3,6);imhist(picture(:,:,:));title('black ');

I want to do this without imhist Is it possible to do this ??