Add tint operation to set image chroma

lib/colour.js

@@ -38,6 +38,21 @@ function background (rgba) {
   return this;
 }
 
+/**
+ * Tint the image using the provided chroma while preserving the image luminance.
+ * An alpha channel may be present and will be unchanged by the operation.
+ *
+ * @param {String|Object} rgb - parsed by the [color](https://www.npmjs.org/package/color) module to extract chroma values.
+ * @returns {Sharp}
+ * @throws {Error} Invalid parameter
+ */
+function tint (rgb) {
+  const colour = color(rgb);
+  this.options.tintA = colour.a();
+  this.options.tintB = colour.b();
+  return this;
+}
+
 /**
  * Convert to 8-bit greyscale; 256 shades of grey.
  * This is a linear operation. If the input image is in a non-linear colour space such as sRGB, use `gamma()` with `greyscale()` for the best results.
@@ -95,6 +110,7 @@ module.exports = function (Sharp) {
   // Public instance functions
   [
     background,
+    tint,
     greyscale,
     grayscale,
     toColourspace,