Use fatih/color for more correct colouring (#17)

## Summary

<!-- Describe your changes in detail here, if it closes an open issue,
include "Closes #<issue>" -->

go.mod

@@ -2,4 +2,13 @@ module github.com/FollowTheProcess/snapshot
 
 go 1.23
 
-require golang.org/x/tools v0.29.0
+require (
+	github.com/fatih/color v1.18.0
+	golang.org/x/tools v0.29.0
+)
+
+require (
+	github.com/mattn/go-colorable v0.1.14 // indirect
+	github.com/mattn/go-isatty v0.0.20 // indirect
+	golang.org/x/sys v0.29.0 // indirect
+)

go.sum

@@ -1,2 +1,15 @@
+github.com/fatih/color v1.18.0 h1:S8gINlzdQ840/4pfAwic/ZE0djQEH3wM94VfqLTZcOM=
+github.com/fatih/color v1.18.0/go.mod h1:4FelSpRwEGDpQ12mAdzqdOukCy4u8WUtOY6lkT/6HfU=
+github.com/mattn/go-colorable v0.1.13 h1:fFA4WZxdEF4tXPZVKMLwD8oUnCTTo08duU7wxecdEvA=
+github.com/mattn/go-colorable v0.1.13/go.mod h1:7S9/ev0klgBDR4GtXTXX8a3vIGJpMovkB8vQcUbaXHg=
+github.com/mattn/go-colorable v0.1.14 h1:9A9LHSqF/7dyVVX6g0U9cwm9pG3kP9gSzcuIPHPsaIE=
+github.com/mattn/go-colorable v0.1.14/go.mod h1:6LmQG8QLFO4G5z1gPvYEzlUgJ2wF+stgPZH1UqBm1s8=
+github.com/mattn/go-isatty v0.0.16/go.mod h1:kYGgaQfpe5nmfYZH+SKPsOc2e4SrIfOl2e/yFXSvRLM=
+github.com/mattn/go-isatty v0.0.20 h1:xfD0iDuEKnDkl03q4limB+vH+GxLEtL/jb4xVJSWWEY=
+github.com/mattn/go-isatty v0.0.20/go.mod h1:W+V8PltTTMOvKvAeJH7IuucS94S2C6jfK/D7dTCTo3Y=
+golang.org/x/sys v0.0.0-20220811171246-fbc7d0a398ab/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.6.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.29.0 h1:TPYlXGxvx1MGTn2GiZDhnjPA9wZzZeGKHHmKhHYvgaU=
+golang.org/x/sys v0.29.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
 golang.org/x/tools v0.29.0 h1:Xx0h3TtM9rzQpQuR4dKLrdglAmCEN5Oi+P74JdhdzXE=
 golang.org/x/tools v0.29.0/go.mod h1:KMQVMRsVxU6nHCFXrBPhDB8XncLNLM0lIy/F14RP588=

internal/colour/colour.go

@@ -2,61 +2,26 @@
 package colour
 
 import (
-	"os"
-	"sync"
+	"github.com/fatih/color"
 )
 
-// ANSI codes for coloured output, they are all the same length so as not to throw off
-// alignment of [text/tabwriter].
-const (
-	codeRed    = "\x1b[0;0031m" // Red, used for diff lines starting with '-'
-	codeHeader = "\x1b[1;0036m" // Bold cyan, used for diff headers starting with '@@'
-	codeGreen  = "\x1b[0;0032m" // Green, used for diff lines starting with '+'
-	codeReset  = "\x1b[000000m" // Reset all attributes
+var (
+	red    = color.New(color.FgRed)
+	header = color.New(color.FgCyan, color.Bold)
+	green  = color.New(color.FgGreen)
 )
 
-// getColourOnce is a [sync.OnceValues] function that returns the state of
-// $NO_COLOR and $FORCE_COLOR, once and only once to avoid us calling
-// os.Getenv on every call to a colour function.
-var getColourOnce = sync.OnceValues(getColour)
-
-// getColour returns whether $NO_COLOR and $FORCE_COLOR were set.
-func getColour() (noColour, forceColour bool) {
-	no := os.Getenv("NO_COLOR") != ""
-	force := os.Getenv("FORCE_COLOR") != ""
-
-	return no, force
-}
-
 // Header returns a diff header styled string.
 func Header(text string) string {
-	return sprint(codeHeader, text)
+	return header.Sprint(text)
 }
 
 // Green returns a green styled string.
 func Green(text string) string {
-	return sprint(codeGreen, text)
+	return green.Sprint(text)
 }
 
 // Red returns a red styled string.
 func Red(text string) string {
-	return sprint(codeRed, text)
-}
-
-// sprint returns a string with a given colour and the reset code.
-//
-// It handles checking for NO_COLOR and FORCE_COLOR.
-func sprint(code, text string) string {
-	noColor, forceColor := getColourOnce()
-
-	// $FORCE_COLOR overrides $NO_COLOR
-	if forceColor {
-		return code + text + codeReset
-	}
-
-	// $NO_COLOR is next
-	if noColor {
-		return text
-	}
-	return code + text + codeReset
+	return red.Sprint(text)
 }