Dear Mingrui,

I believe that the addition of color would be very helpful for effective visualization. 

Yes of course, the color value for the nodes having both positive and negative is the critical thing to be considered here. In my opinion, there could be some ways that can be applied:

i) Counting the number of positive and negative edges to a particular node. In simple, the color can be biased to the highest number of edges (either positive or negative). In complex, one way could be to evaluate the intermediate colors based on the edge densities (positive and negative).

ii) If we have significant size of nodes then we can resemble the densities of positive and negative edges in terms of pie-chart as well.

These are just my suggestions to you, and hope it would help you to some extent.

Thank you for your kind response.

Sincerely yours,
Gyan