How to add SwatchLegend to a graph?

You can use Legended:

Legended[g1, legend]

enter image description here

Alternatively, you can use the option Epilog:

RandomGraph[{100, 500}, VertexStyle -> color,  VertexSize -> 1.5, 
 Epilog -> Inset[legend, Scaled @ {1.1, .5}], ImagePadding -> 40]

enter image description here

In general, you can use any type of legends inside the plotting routines:

Plot[
 {Sin@x, Cos@x}, {x, -Pi, Pi},
 PlotStyle -> {Red, Blue},
 PlotLegends -> 
  Placed[SwatchLegend[{Red, Blue}, {"Sin", "Cos"}], Right]
 ]

However, the kglr's way allows making legends for any objects (like arbitrary graphics).

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