You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: colour.go
    path of file: ./repos/wails/v2/internal/colour
    
    the code of the file until where you have to start completion: package colour

import (
	"fmt"
	"strings"

	"github.com/wzshiming/ctc"
)

var ColourEnabled = true

fu
    
func Rainbow(text string) string {
	if !ColourEnabled {
		return text
	}
	var builder strings.Builder

	for i := 0; i < len(text); i++ {
		fn := rainbowCols[i%len(rainbowCols)]
		builder.WriteString(fn(text[i : i+1]))
	}

	return builder.String()
}