How to covert back the gray scale image into rgb?

I am trying to crop the image using its pixel value and the n convert into rgb again but when i get back the rgb image its in the yellow back ground.

img1 = imread('snr30i4ASK100.png');  % imread( ) can load images of many formats
img1 = rgb2gray(img1);
row_i = find(any(img1<255,2));
first_rowi = row_i(1);
last_rowi = row_i(end);
col_i = find(any(img1<255,1));
first_coli = col_i(1);
last_coli = col_i(end);
new_imgi = img1(first_rowi:last_rowi, first_coli:last_coli);
rgbImage = ind2rgb(new_imgi, colormap);

I also upload the input image and its output

Answer 1

[yanqi liu]

yes，sir，may be check the colormap，such as

close all;
clear all;
clc;
img1 = imread('https://ww2.mathworks.cn/matlabcentral/answers/uploaded_files/824690/snr30i4ASK100.png');  % imread( ) can load images of many formats
imgt = img1;
img1 = rgb2gray(img1);
row_i = find(any(img1<255,2));
first_rowi = row_i(1);
last_rowi = row_i(end);
col_i = find(any(img1<255,1));
first_coli = col_i(1);
last_coli = col_i(end);
new_imgi = img1(first_rowi:last_rowi, first_coli:last_coli);
% rgbImage = ind2rgb(new_imgi, colormap);
bw=new_imgi>180;
rgbImage = ind2rgb(bw, [0 0 1;1 1 1]);
figure; imshow(mat2gray(rgbImage))

rgbImage2 = imgt(first_rowi:last_rowi, first_coli:last_coli,:);
figure; imshow(mat2gray(rgbImage2))

[john karli]

can you please explain the following code

bw=new_imgi>180;
rgbImage = ind2rgb(bw, [0 0 1;1 1 1]);
rgbImage2 = imgt(first_rowi:last_rowi, first_coli:last_coli,:);

[john karli]

any why you use mat2gray(rgbImage2). if we use just imshow(rgbImage2) it will same as mat2gray(rgbImage2)

[yanqi liu]

yes，sir

bw=new_imgi>180; % make the image to black and white

rgbImage = ind2rgb(bw, [0 0 1;1 1 1]); % make 0 and 1 to color blue and white

rgbImage2 = imgt(first_rowi:last_rowi, first_coli:last_coli,:); % this is to crop the origin image

mat2gray(rgbImage2). if we use just imshow(rgbImage2) it will same as mat2gray(rgbImage2)

use it to display in [0,1] range