How to build a 3D matrix?

I have a homework assignment that I just need help understanding. We are to make a 4x6x3 matrix, where each column of the image is a single color, in rainbow order (Red, Orange, Yellow, Green, Blue, Purple), but we must use for loops. I'm not quite sure what I'm doing, but I did write a code.

mat=zeros(4,6,3);
row1=[255,0,0];
row2=[0,255,0];
row3=[0,0,255];
for 
    img(:,col,1)=mycolormap(col,1);
    img(:,col,2)=mycolormap(col,2);
    img(:,col,3)=mycolormap(col,3);
end

please help me out. I'm new to Matlab and I'm little confused.

[John BG]

you mean a volume, you want to build a volume?

[Sarah Sadeq]

I'd like to build Build a 4x6x3 matrix, where each column of the$ mage is a single color, in rainbow order

Answer 1

[Walter Roberson]

You are close in your approach. You would need

mycolormap = [row1; row2; row3];   %(and extend this for the 6 different colors)

And you would need to fix your for loop.