You need to turn pos into integers instead of fractional values.
Try this:
close all;
clear all;
clc;
rgbImage = imread('sevilla.jpg');
figure('Name','Sevilla');
imshow(rgbImage)
axis('on', 'image')
uiwait(helpdlg('Draw a rectangle'))
roi = drawrectangle('StripeColor','r')
pos = roi.Position
% OPTIONAL Get rid of graphical overlay and replace with yellow rectangle.
delete(roi)
rectangle('Position', pos, 'EdgeColor', 'y', 'LineWidth', 2);
% Crop image using indexing.
col1 = floor(pos(1)); % Column 1
col2 = ceil(pos(1) + pos(3)); % Column 2
row1 = floor(pos(2)); % Row 1
row2 = ceil(pos(2) + pos(4)); % Row 2
croppedImage = rgbImage(row1 : row2, col1 : col2, :);
figure
imshow(croppedImage)
axis('on', 'image')
