Palette coloring for multi-graphs is activated thanks to the options PFC (Palette Fill Color), PLC (Palette Line Color) and AMC (Palette Marker Color).
When one of these options is given to TMultiGraph::Draw the TGraphs in the TMultiGraph get their color from the current color palette defined by gStyle->SetPalette(...). The color is determined according to the number of TGraphs.
In this example four graphs are displayed with palette coloring for lines and and markers. The color of each graph is picked inside the default palette kBird.
Definition in file gr105_multigraphpalettecolor.C.