How to convert a RGB image to a two dimensional image in Matlab

I have a RGB image called imrgb, the size of which is 320*512*3. I want to know how to convert it to a two dimensional image?
I saw some colorful images, but they are still two dimensional instead of three dimensional. Can anyone tell me how to convert it to a two-dimensional image but still looks colorful?

If I understood right you can store the image data as a 2D array and use a colormap to assign it colors, displaying it as a "colorful" image. In this case the colormap will be a Nx3 array where each row corresponds to an index present (or not) in the image data. So if the colormap is a 256x3 array, for example, the image will be made of indices ranging from 0 to 255.

For example, let's consider the peppers.png demo image that ships with Matlab. Let's read and use the rgb2ind function to store the image data in variable X and its associated colormap in variable map.

a = imread('peppers.png');
[X,map] = rgb2ind(a,256);

If we ask the size of X, we get:

ans =

   384   512

So a 2D variable, even though the original image (a) was 3D (rgb).

If we wish to display this 2D data as a colorful we need to provide a suitable colormap. For instance, we can use the original colormap (stored in map) in order to get the following:

image(X)
colormap(map)
axis off

or we can use any other colormap of our choice.

For example:

image(X)
colormap(jet(200)) %// The size can be different than the original colormap
axis off

yielding:

So as you see the image looks 3D but the data is actually 2D.

Is this what you meant?