diff --git a/main.go b/main.go
index 5677d3e..4376585 100644
--- a/main.go
+++ b/main.go
@@ -166,26 +166,19 @@ func (c *HSV) to_rgb() *RGB {
 	return f2rgb(0, 0, 0)
 }
 
-func add255(x uint8, y uint8) uint8 {
-	if x < 255-y {
-		return x + y
+func (c *RGB) brighterThan(ref RGB, delta uint8) string {
+	primary := c.to_hsv()
+	secondary := ref.to_hsv()
+	delta_f := float64(delta) * 1.0 / 255.0
+	if primary.v >= secondary.v+delta_f {
+		return fmt.Sprintf("#%02x%02x%02x", c.r, c.g, c.b)
 	}
-	return 255
-}
-
-func sub255(x uint8, y uint8) uint8 {
-	if x > y {
-		return x - y
+	primary.v = secondary.v + delta_f
+	if primary.v > 360 {
+		primary.v = 360
 	}
-	return 0
-}
-
-func (c *RGB) brighterOrDarkerThan(ref RGB, delta uint8) string {
-	/* XXX FIXME: THIS IS PROBABLY NOT CORRECT. Need to look into original implementation. */
-	if true {
-		return fmt.Sprintf("#%02x%02x%02x", add255(c.r, delta), add255(c.g, delta), add255(c.b, delta))
-	}
-	return ref.darkerThan(ref, delta)
+	c = primary.to_rgb()
+	return fmt.Sprintf("#%02x%02x%02x", c.r, c.g, c.b)
 }
 
 func (c *RGB) darkerThan(ref RGB, delta uint8) string {
@@ -203,6 +196,17 @@ func (c *RGB) darkerThan(ref RGB, delta uint8) string {
 	return fmt.Sprintf("#%02x%02x%02x", c.r, c.g, c.b)
 }
 
+func (c *RGB) brighterOrDarkerThan(ref RGB, delta uint8) string {
+	primary := c.to_hsv()
+	secondary := ref.to_hsv()
+
+	if primary.v <= secondary.v {
+		return c.darkerThan(ref, delta)
+	} else {
+		return c.brighterThan(ref, delta)
+	}
+}
+
 func (c RGB) withAlpha(alpha float64) string {
 	return fmt.Sprintf("#%02x%02x%02x%02x", c.r, c.g, c.b, uint8(255*alpha))
 }