New to AP,  trying to figure out how to edit the alpha channel of a TIFF file. In the Channels panel, I click Composite Alpha to make the alpha channel editable, but some tools seem to work inconsistently or not at all in the alpha channel. For example, I can select all and Fill the alpha channel with white, but not with black or gray. Same with the Paint Brush tool. And it won't let me paste into the channel, pasting with the alpha channel editable and the other channels set to non-editable, th