nodes: use different colors from config based on category

app/render/color.cpp

@@ -207,6 +207,11 @@ QColor Color::toQColor() const
   return c;
 }
 
+float Color::GetRoughLuminance() const
+{
+  return (2*red()+blue()+3*green())/6.0f;
+}
+
 const Color &Color::operator+=(const Color &rhs)
 {
   for (int i=0;i<kRGBAChannels;i++) {

app/render/color.h

@@ -86,6 +86,10 @@ public:
 
   QColor toQColor() const;
 
+  // Suuuuper rough luminance value mostly used for UI (determining whether to overlay with black
+  // or white text)
+  float GetRoughLuminance() const;
+
   // Assignment math operators
   const Color& operator+=(const Color& rhs);
   const Color& operator-=(const Color& rhs);