add function to calculate grayscale of a color

src/utils.js

@@ -130,6 +130,19 @@ export function getColor ({red, green, blue, alpha}) {
 		`rgb(${ red }, ${ green }, ${ blue })`;
 }
 
+const GRAY_FACTOR = {
+	red: 0.299,
+	green: 0.587,
+	blue: 0.114,
+};
+
+// grayscale accounting for luminosity
+export function grayscale ({red, green, blue}) {
+	return GRAY_FACTOR.red * red
+		+ GRAY_FACTOR.green * green
+		+ GRAY_FACTOR.blue * blue;
+}
+
 export function extend (obj, ...sources) {
 	return Object.assign({}, obj, ...sources);
 }