An answer of the exercise: Images on a tour of go

images.go

package main

import (
    "code.google.com/p/go-tour/pic"
    "image"
    "image/color"
)

type Image struct {
    Width, Height int
    Color uint8
}

func (i *Image) ColorModel() color.Model {
    return color.RGBAModel
}

func (i *Image) Bounds() image.Rectangle {
    return image.Rect(0, 0, i.Width, i.Height)
}

func (i *Image) At(x, y int) color.Color {
    return color.RGBA{i.Color, i.Color, 255, 255}
}

func main() {
    m := &Image{100, 100, 128}
    pic.ShowImage(m)
}