Use Merge and Open for multichannel confocal image stacks.
If you use Open>Image Stack instead of this command, the images open without merging.
To open a multichannel image stack with each channel displayed in a different color:
The same file name appears in each Image Stack field.
To assign a color to a file, select the correct file in the Image Stack drop-downs.
Leave the Image Channel fields blank since each file contains only one channel.
If the Image Scaling and Placement window appears, see instructions here.