-Create a regular RGB document. It has to match the the size of the target document, i.e. 2048 and 2048.
-Create 3 layers. For the sake of the tutorial name them identical to the channel you will be transferring them to, so red, green, blue.
-In each layer, you will create your mask. Be sure to fill the mask with pure white.
-Ctrl + Click your layer’s icon to grab the selection (Be sure your layer is still selected, or else it won’t work) and Ctrl + C to copy.
-Go into your target mask document. This should be the same size as your original document, and should have a black background. Go over to the channels tab and select the red channel, go to edit > paste > paste in place. It should paste it exactly where it belongs in your new document, and isolate it to the red channel.
-Repeat these steps for Green and Blue. You can also create an alpha channel to store a 4th mask.
-When you are finished, select RGB to view the image as a whole and it should be a mixture of colors. Save the final composition, and then plug in those values in UE4.
Let me know if you need any more clarification. =)