diff --git a/captcha.go b/captcha.go
index ce7d7c4..c83901f 100644
--- a/captcha.go
+++ b/captcha.go
@@ -12,6 +12,7 @@ import (
 	"io"
 	"math"
 	"math/rand"
+	"strconv"
 	"time"
 )
 
@@ -193,3 +194,63 @@ func drawText(text string, img *image.NRGBA, opts *Options) error {
 
 	return nil
 }
+
+func randomInvertColor(base color.Color) color.Color {
+	baseLightness := getLightness(base)
+	var value float64
+	if baseLightness >= 0.5 {
+		value = baseLightness - 0.25 - rng.Float64()*0.2
+	} else {
+		value = baseLightness + 0.25 + rng.Float64()*0.2
+	}
+	hue := float64(rng.Intn(361)) / 360
+	saturation := 0.6 + rng.Float64()*0.2
+
+	return hsva{h: hue, s: saturation, v: value, a: uint8(255)}
+}
+
+func getLightness(colour color.Color) float64 {
+	r, g, b, a := colour.RGBA()
+	// transparent
+	if a == 0 {
+		return 1.0
+	}
+	max := maxColor(r, g, b)
+	min := minColor(r, g, b)
+
+	l := (float64(max) + float64(min)) / (2 * 255)
+
+	return l
+}
+
+func maxColor(numList ...uint32) (max uint32) {
+	for _, num := range numList {
+		colorVal := num & 255
+		if colorVal > max {
+			max = colorVal
+		}
+	}
+
+	return max
+}
+
+func minColor(numList ...uint32) (min uint32) {
+	min = 255
+	for _, num := range numList {
+		colorVal := num & 255
+		if colorVal < min {
+			min = colorVal
+		}
+	}
+
+	return min
+}
+
+func randomEquation() (text string, equation string) {
+	left := 1 + rng.Intn(9)
+	right := 1 + rng.Intn(9)
+	text = strconv.Itoa(left + right)
+	equation = strconv.Itoa(left) + "+" + strconv.Itoa(right)
+
+	return text, equation
+}
diff --git a/captcha_test.go b/captcha_test.go
index 5f7383c..5e4c3fd 100644
--- a/captcha_test.go
+++ b/captcha_test.go
@@ -5,6 +5,7 @@ import (
 	"golang.org/x/image/font/gofont/goregular"
 	"image/color"
 	"testing"
+	"math/rand"
 )
 
 func TestNewCaptcha(t *testing.T) {
@@ -55,3 +56,49 @@ func TestLoadFont(t *testing.T) {
 		t.Fatal("LoadFont incorrectly parse an invalid font")
 	}
 }
+
+func TestMaxColor(t *testing.T) {
+	var result uint32
+	result = maxColor()
+	if result != 0 {
+		t.Fatalf("Expect max color to be 0, got %v", result)
+	}
+	result = maxColor(1)
+	if result != 1 {
+		t.Fatalf("Expect max color to be 1, got %v", result)
+	}
+	result = maxColor(52428, 65535)
+	if result != 255 {
+		t.Fatalf("Expect max color to be 255, got %v", result)
+	}
+	var rng = rand.New(rand.NewSource(0))
+	for i := 0; i < 10; i++ {
+		result = maxColor(rng.Uint32(), rng.Uint32(), rng.Uint32())
+		if result > 255 {
+			t.Fatalf("Number out of range: %v", result)
+		}
+	}
+}
+
+func TestMinColor(t *testing.T) {
+	var result uint32
+	result = minColor()
+	if result != 255 {
+		t.Fatalf("Expect min color to be 255, got %v", result)
+	}
+	result = minColor(1)
+	if result != 1 {
+		t.Fatalf("Expect min color to be 1, got %v", result)
+	}
+	result = minColor(52428, 65535)
+	if result != 204 {
+		t.Fatalf("Expect min color to be 1, got %v", result)
+	}
+	var rng = rand.New(rand.NewSource(0))
+	for i := 0; i < 10; i++ {
+		result = minColor(rng.Uint32(), rng.Uint32(), rng.Uint32())
+		if result > 255 {
+			t.Fatalf("Number out of range: %v", result)
+		}
+	}
+}