color_combination

How to Create Duotones in Photoshop
http://www.wikihow.com/Create-Duotones-in-Photoshop

How to Use Gradient Maps to Create A Cool Duotone Effect In Photoshop
https://audiotheme.com/blog/how-to-use-gradient-maps-to-create-a-cool-duotone-effect-in-photoshop/

SwatchSpot creates random color swatches

http://swatchspot.com/

http://www.colourlovers.com/

http://digitalphenom.deviantart.com/art/Complementary-Color-SwatchesII-111331715

photoshop action download

https://www.dropbox.com/s/u1zqwlnlpg5zovk/Two_Tone_Fade_Actions_by_BakaArts.atn?dl=1