If you are trying to store 15 RGB images of size 64 x 128 x 3, the tran_x should have the size [64,128,3,15], exactly like you have it. You may also want to make it a uint8 array from the start, like this:
train_x = zeros([64,128,3,15], 'uint8');
If you want to store 15 grayscale images of size 64 x 128, then train_x should have the size of [64, 128, 15].
If some of your images are RGB and some are grayscale, then there are several things you can do. You could make all images RGB.
imgRGB = cat(3, imgGray, imgGray, imgGray);
Then all of your images will have the size 64x128x3. Alternatively, you can use a cell array to store the images. Elements of a cell array can have different sizes and even data types.
