diff --git a/colour.cpp b/colour.cpp
index acc55084..22d4df07 100644
--- a/colour.cpp
+++ b/colour.cpp
@@ -3,13 +3,17 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/.
  *
- * Copyright 2010-2017 Danny Robson <danny@nerdcruft.net>
+ * Copyright 2010-2021 Danny Robson <danny@nerdcruft.net>
  */
 
 #include "colour.hpp"
 
 #include "ascii.hpp"
 #include "parse/value.hpp"
+#include "debug/assert.hpp"
+
+using cruft::colour;
+using cruft::srgba;
 
 
 ///////////////////////////////////////////////////////////////////////////////
@@ -48,4 +52,156 @@ cruft::srgba4f
 cruft::parse::value<cruft::srgba4f> (cruft::view<const char*> &str)
 {
     return parse_hex (str);
+}
+
+
+///////////////////////////////////////////////////////////////////////////////
+/// Invert the sRGB gamma function
+static constexpr float
+linearise (float const u)
+{
+    if (u <= 0.04045f)
+        return u / 12.92f;
+    else
+        return std::pow ((u + 0.055f) / 1.055f, 2.4f);
+}
+
+
+///----------------------------------------------------------------------------
+/// Apply the sRGB gamma function
+static constexpr float
+exponentiate (float const u)
+{
+    if (u <= 0.0031308f)
+        return 12.92f * u;
+    else
+        return 1.055f * std::pow (u, 1/2.4f) - 0.055f;
+}
+
+
+///----------------------------------------------------------------------------
+/// Convert an sRGB colour to XYZ.
+///
+/// Gamma will be removed during the conversion. Do not use with linear RGB.
+static cruft::vector3f
+sRGB_to_XYZ (cruft::vector3f sRGB)
+{
+    auto const [r_, g_, b_] = sRGB;
+
+    float const r = linearise (r_);
+    float const g = linearise (g_);
+    float const b = linearise (b_);
+
+    // D65 whitepoint, 2° observer.
+    //
+    // We use higher precision values than the specification dictates, from
+    // Bruce Lindbloom's reference, so as to minimise numerical errors when
+    // chaining conversions.
+    float const X = r * 0.4124564f + g * 0.3575761f  + b * 0.1804375f;
+    float const Y = r * 0.2126729f + g * 0.7151522f  + b * 0.0721750f;
+    float const Z = r * 0.0193339f + g * 0.1191920f  + b * 0.9503041f;
+
+    return {X, Y, Z};
+}
+
+
+///----------------------------------------------------------------------------
+/// Convert an XYZ colour to sRGB.
+///
+/// Gamma will be applied during the conversion.
+///
+/// Out of range colours will be returned as is. It is the caller's
+/// responsibility to process these as desired.
+static cruft::vector3f
+XYZ_to_sRGB (cruft::vector3f XYZ)
+{
+    auto const [X, Y, Z] = XYZ;
+
+    float const r = +3.2404542f * X -1.5371385f * Y -0.4985314f * Z;
+    float const g = -0.9692660f * X +1.8760108f * Y +0.0415560f * Z;
+    float const b = +0.0556434f * X -0.2040259f * Y +1.0572252f * Z;
+
+    return {
+        exponentiate (r),
+        exponentiate (g),
+        exponentiate (b),
+    };
+}
+
+
+///----------------------------------------------------------------------------
+/// Convert an XYZ colour to xyY
+static
+cruft::vector3f
+XYZ_to_xyY (cruft::vector3f XYZ)
+{
+    auto const [X, Y, Z] = XYZ;
+
+    if (cruft::abs (X + Y + Z) < 1e-5f) {
+        // Use the D65 xy if we're at black.
+        return {
+            0.31271f,
+            0.32902f,
+            Y
+        };
+    } else {
+        return {
+            X / (X + Y + Z),
+            Y / (X + Y + Z),
+            Y,
+        };
+    }
+}
+
+
+///----------------------------------------------------------------------------
+/// Convert an xyY colour to XYZ
+static cruft::vector3f
+xyY_to_XYZ (cruft::vector3f xyY)
+{
+    auto const [x, y, Y] = xyY;
+    if (cruft::abs (y) < 1e-5f) {
+        return {0, 0, 0};
+    } else {
+        return {
+            x * Y / y,
+            Y,
+            (1 - x - y) * Y / y,
+        };
+    }
+}
+
+
+//-----------------------------------------------------------------------------
+cruft::srgba3f
+cruft::scale_luminance (cruft::srgba3f sRGB, float f)
+{
+    // We can't scale sRGB directly. Given we probably have to convert to other
+    // spaces via XYZ anyway we may as well scale the luminance value in xyY.
+    //
+    // The conversion chain is thus:
+    //    * sRBB -> XYZ -> xyY -> scaling -> XYZ -> sRGB
+
+    auto const XYZ0= sRGB_to_XYZ (sRGB.as<cruft::vector3f> ());
+    auto const xyY0= XYZ_to_xyY (XYZ0);
+
+    auto const xyY1 = xyY0 * cruft::vector3f{1,1,f};
+    auto const XYZ1 = xyY_to_XYZ (xyY1);
+    return XYZ_to_sRGB (XYZ1).as<cruft::srgba3f> ();
+}
+
+
+//-----------------------------------------------------------------------------
+cruft::srgba4f
+cruft::scale_luminance (cruft::srgba4f sRGBA, float f)
+{
+    auto const sRGB0 = sRGBA.indices<0,1,2> ();
+    auto const sRGB1 = scale_luminance (sRGB0, f);
+
+    return {
+        sRGB1.r,
+        sRGB1.g,
+        sRGB1.b,
+        sRGBA.a,
+    };
 }
\ No newline at end of file
diff --git a/colour.hpp b/colour.hpp
index e1d2120e..ff916fbd 100644
--- a/colour.hpp
+++ b/colour.hpp
@@ -84,6 +84,9 @@ namespace cruft {
     using srgba3u = srgba<3,uint8_t>;
     using srgba4u = srgba<4,uint8_t>;
 
+    srgba3f scale_luminance (srgba3f, float factor);
+    srgba4f scale_luminance (srgba4f, float factor);
+
     template <size_t S, typename T> struct hsva  : colour<S,T,hsva<S,T>> {};
 
     template <size_t S, typename T>
diff --git a/test/colour.cpp b/test/colour.cpp
index 187d50c2..3afed11b 100644
--- a/test/colour.cpp
+++ b/test/colour.cpp
@@ -16,5 +16,15 @@ main (int, char**)
         tap.expect_eq (u.cast<float> (), f, "cast u8 to float");
     }
 
+    {
+        cruft::srgba3f white {1, 1, 1};
+        cruft::srgba3f black {0, 0, 0};
+        cruft::srgba3f percent_50 { 0.735359f, 0.735356f, 0.735357f };
+
+        tap.expect (max (scale_luminance (white, 1.f) - white) < 1e-5f, "unit luminance scaling for white");
+        tap.expect (max (scale_luminance (white, 0.f) - black) < 1e-5f, "zero luminance scaling for white");
+        tap.expect (max (scale_luminance (white, .5f) - percent_50) < 1e-5f, "half luminance scaling for white");
+    }
+
     return tap.status ();
 }