Merge layers and fill gaps

I have 14 layers, which are polygons. They represent the channel of a river at different periods. I need to fill the gaps that exist once I overlay the polygons, but I can't change the properties of the original layers because I need to reclassify the resultant polygon on the base of the colour in order to do more analysis. I have tried to merge all 14 polygons, but the new polygon doesn't have the same properties of each individual polygon.

Do you know how I could fill the gaps between the 14 polygons and some way to merge all the polygons keeping the same characteristics of the individual layers so I can reclassify the resultant polygon by colour? Thanks!