Hopefully this will give you a better recipe than "season to taste!"
First attachment is the three source panels from the single negative linked above in a previous post (Library of Congress). The assumption is that red, green and blue filters were used for the separation, so, if we make some assumptions about the subject, we can infer which plate is which. If we assume a caucasian-like skin tone, then blue will give us a dark tonal value for skin, green a mid tone and red a light tone. This helps as a first pass guess for the recombination of the separate plates in the negative
Second attachment is a stack using Photo Filters. The key to the adjustment layer approach is to make sure that each adjustment layer is CLIPPED to the B&W image below it so that the adjustment (gradient, photo filter, etc.) will only affect the layer directly below it and not the whole stack. For anyone using PS that is not aware of how to clip the layer, make sure the adjustment layer is directly above the layer you want to operate on, hold down the Option (ALT on PC) key and hoever over the border between the two layers in the layers panel - the cursor will change to an "L" with a downward pointing arrow - click on the border and the adjustment layer will become indented in the stack, with the arrow to its left indicating that the layer is clipped to the one below it.
Photo filter settings: 100% density (or season to taste) and uncheck "preserve luminosity". Filter layer blend mode: normal, 100% opacity.
Layer blend modes: bottom layer: normal, 100% opacity - middle and top layers: linear dodge (add), 100% opacity.
Give this a shot and see if it works. I understand this is more complicated than necessary for the approach where you extract the channels from a color image using the B&W representation in the PS Channels panel. In that case, all plates are registered and the channel color is exactly known. However, if you want to composite Prokudin-Gorsky' type negatives, you may want a little more control over the compositing process.
Hope this helps!