8160eae2____GeoServer_2_7_x_User_Manual__Description
[Parameter] ExpandToRGB [Mandatory] N [Description] 
Applies to colormapped granules. Asks the internal mosaic engine to expand the colormapped granules to RGB prior to mosaicking them. This is needed whenever the the granules do not share the same color map hence a straight composition that would retain such a color map cannot be performed.
Using the ImageMosaic plugin — GeoServer 2.7.x User Manual