See this snippet where I check if the number of color channels is 1 and then, if it's not, and I wanted a grayscale image instead of an RGB image, I convert it to gray scale:
grayImage = imread(fullFileName);
[rows, columns, numberOfColorChannels] = size(grayImage);
if numberOfColorChannels > 1
grayImage = grayImage(:, :, 2);
end
Of course you can call the image array anything you want, and check if numberOfColorChannels > 1 or if numberOfColorChannels == 3, or however you want to do it, and then take whatever actions you want to based on knowing how many color channels it has.