Stevens, Joshua (2015) Bivariate Choropleth Maps: A How-to Guide, retrived at Nov 1st, 2017, http://www.joshuastevens.net/cartography/make-a-bivariate-choropleth-map/
https://www.w3schools.com/colors/default.asp
Other related:
http://www.personal.psu.edu/cab38/ColorSch/SchHTMLs/CBColorSeqSeq.html
http://www.personal.psu.edu/cab38/ColorSch/Schemes.html
http://tolomaps.tumblr.com/post/131671267233/creating-a-bivariate-choropleth-color-scheme
glow effects: http://www.joshuastevens.net/visualization/squatch-watch-92-years-of-bigfoot-sightings-in-us-and-canada/