Add color.tint and color.shade operations

These produce tints and shades of the color, i.e. mix with either white
or black at a given ratio.

See https://en.wikipedia.org/wiki/Tints_and_shades

Closes gka#235

docs/src/index.md

@@ -386,6 +386,32 @@ chroma('hotpink').mix('blue', 0.25);
 chroma('hotpink').mix('blue', 0.75);
 ```
 
+### color.shade
+#### (ratio=0.5)
+
+Produce a shade of the color.
+This is syntactic sugar for `color.mix` with a target color of black.
+
+```js
+chroma('hotpink').shade();
+chroma('hotpink').shade(0.25);
+chroma('hotpink').shade(0.75);
+chroma('hotpink').shade(1);
+```
+
+### color.tint
+#### (ratio=0.5)
+
+Produce a tint of the color.
+This is syntactic sugar for `color.mix` with a target color of white.
+
+```js
+chroma('hotpink').tint();
+chroma('hotpink').tint(0.25);
+chroma('hotpink').tint(0.75);
+chroma('hotpink').tint(1);
+```
+
 
 ### color.set
 #### (channel, value)

index-light.js

@@ -16,6 +16,8 @@ require('./src/ops/darken');
 require('./src/ops/get');
 require('./src/ops/mix');
 require('./src/ops/set');
+require('./src/ops/shade');
+require('./src/ops/tint');
 
 // interpolators
 require('./src/interpolator/lrgb');

index.js

@@ -29,6 +29,8 @@ require('./src/ops/mix');
 require('./src/ops/premultiply');
 require('./src/ops/saturate');
 require('./src/ops/set');
+require('./src/ops/shade');
+require('./src/ops/tint');
 
 // interpolators
 require('./src/interpolator/rgb');

src/ops/shade.js

@@ -0,0 +1,6 @@
+const Color = require('../Color');
+const mix = require('../generator/mix');
+
+Color.prototype.shade = function(f=0.5, ...rest) {
+	return mix(this, 'black', f, ...rest);
+}

src/ops/tint.js

@@ -0,0 +1,6 @@
+const Color = require('../Color');
+const mix = require('../generator/mix');
+
+Color.prototype.tint = function(f=0.5, ...rest) {
+	return mix(this, 'white', f, ...rest);
+}

test/manipulate.test.js

@@ -21,7 +21,13 @@ vows
             'saturate'(topic) { return assert.equal(topic.saturate().hex(), '#ff0000'); },
             'desaturate'(topic) { return assert.equal(topic.desaturate().hex(), '#ee3a20'); },
             'desaturate more'(topic) { return assert.equal(topic.desaturate(2).hex(), '#db5136'); },
-            'desaturate too much'(topic) { return assert.equal(topic.desaturate(400).hex(), '#7f7f7f'); }
+            'desaturate too much'(topic) { return assert.equal(topic.desaturate(400).hex(), '#7f7f7f'); },
+            'shade'(topic) { return assert.equal(topic.shade().hex(), '#b40000'); },
+            'shade more'(topic) { return assert.equal(topic.shade(0.75).hex(), '#800000'); },
+            'shade too much'(topic) { return assert.equal(topic.shade(1.5).hex(), '#000000'); },
+            'tint'(topic) { return assert.equal(topic.tint().hex(), '#ffb4b4'); },
+            'tint more'(topic) { return assert.equal(topic.tint(0.75).hex(), '#ffdddd'); },
+            'tint too much'(topic) { return assert.equal(topic.tint(1.5).hex(), '#ffffff'); }
         },
 
         'premultiply': {