how to index a 3D image

Hi, I have an RGB tif image (no map) of some color bars (it wouldn't let me upload a tif). It is 77x300x30, each of the color bars is approximately 20 columns of data. I have taken the middle column out of each of the color bars and saved them to a new array, and then show it as an image. I tried a for loop and got nowhere with it, now I am trying to index it with an array but I am sure there is a more straightforward method than the longhand way I have used, any ideas?

   clear all
    close all
    clc

     img = imread('spectrum11.jpg');
     imwrite(img, 'spectrum11.tiff');

    %%==============================================
    [B,map]=imread('spectrum11.tiff');
    slice=B(1:10,:,:);
    subplot(3,1,1)
    imshow(B)
    title('original spectrum11 pic')
    subplot(3,1,2)
    imshow(slice)
    title('sliver of the spectrum')

    %show a few rows of the image
  slice_data=slice(1,:,:);
  [r,c,d]=size(slice_data)

    %%take the middle 5 columns of each colour bar from the slice_data.
    %preallocate an array
    spec=zeros(1,45,3);

    %get an index for the middle of each color bar
    ind=zeros(15,1,3)
    ind_nums=(10:20:300)'
    ind(:,1,1)=ind_nums;
    ind(:,1,2)=ind_nums;
    ind(:,1,3)=ind_nums;

    %use index to extract the data from each of the RGB layers of slice_data
    slice_data_1=slice_data(:,:,1);
  slice_data_2=slice_data(:,:,2);
  slice_data_3=slice_data(:,:,3);

A1=slice_data_1(ind(:,:,1));
A2=slice_data_2(ind(:,:,2));
A3=slice_data_3(ind(:,:,3));

A=zeros(1,15,3);

A(:,:,1)=A1;
A(:,:,2)=A2;
A(:,:,3)=A3;

Au=uint8(A);

subplot(3,1,3)
imshow(Au)

Answer 1

[Yuvaraj Venkataswamy]

@Stephen Devlin: For this also, you can see the below link.

https://in.mathworks.com/help/images/examples/enhancing-multispectral-color-composite-images.html