Each image is a matrix. An RGB image is a 3d matrix of size (height, width, depth). The depth will be 3, i.e. red, green and blue.
If you know the coordinates of your points of interest, you can simply select those elements in the matrix around that point, like this:
m = imread('peppers.png');
imshow(m)
% get a point of interest (poi)
x = 250;
y = 250;
sides = 30;
region_of_interest = m(x-sides:x+sides, y-sides:y+sides, :);
imshow(region_of_interest)
% average pixel colour as an RGB triplet
average_pixel_colour = mean(region_of_interest, [1 2])
% if you want the whitest pixel value, then this may be what you are looking for:
greyscale_region_of_interest = mean(region_of_interest, [3]); % you could use also rgb2gray instead
whitest_pixel_value = max(greyscale_region_of_interest, [], 'all')
