The GML graph files can have colors for nodes and edges using fill statement and html #rrggbb color like this:

#colors in nodes/edges using 'fill'

graph [

directed 1

node [ id 0 label "n0" graphics [ fill "#f00000" ] ]

node [ id 1 label "n1" graphics [ fill "#00ff00" ] ]

node [ id 2 label "n2" graphics [ fill "#f0ff00" ] ]

node [ id 3 label "n3" graphics [ fill "#f08000" ] ]

node [ id 4 label "n4" graphics [ fill "#f0000f" ] ]

edge [ source 4 target 0 graphics [ fill "#f000ff" ] ]

edge [ source 0 target 1 label "foo" graphics [ fill "#0f0ff0" ] ]

edge [ source 1 target 2 graphics [ fill "#abf000" ] ]

edge [ source 2 target 3 graphics [ fill "#f0f000" ] ]

edge [ source 3 target 0 graphics [ fill "#f000f0" ] ]

edge [ source 3 target 0 graphics [ fill "#ff000b" ] ]

]