One-Color

main.go

package main

import (
        "image"
        "image/color"
        "image/png"
        "os"
)

var baseColor = color.RGBA{0x65, 0x73, 0x78, 0xff}

func run(in string, out string) error {
        f, err := os.Open(in)
        if err != nil {
                return err
        }
        defer f.Close()

        img, err := png.Decode(f)
        if err != nil {
                return err
        }

        outimg := image.NewRGBA(img.Bounds())

        w, h := img.Bounds().Dx(), img.Bounds().Dy()
        brf := float64(baseColor.R) / 0xff
        bgf := float64(baseColor.G) / 0xff
        bbf := float64(baseColor.B) / 0xff

        for j := 0; j < h; j++ {
                for i := 0; i < w; i++ {
                        r, _, _, a := img.At(i, j).RGBA()
                        if a == 0 {
                                continue
                        }
                        rf := float64(r) / 0xffff
                        af := float64(a) / 0xffff

                        c := color.NRGBA{
                                uint8(((1 - rf) * brf + rf) * 0xff),
                                uint8(((1 - rf) * bgf + rf) * 0xff),
                                uint8(((1 - rf) * bbf + rf) * 0xff),
                                uint8(af * 0xff)}
                        outimg.Set(i, j, c)
                }
        }

        of, err := os.Create(out)
        if err != nil {
                return err
        }
        defer of.Close()
        if err := png.Encode(of, outimg); err != nil {
                return err
        }

        return nil
}

func main() {
        if err := run(os.Args[1], os.Args[2]); err != nil {
                panic(err)
        }
}