Your color models return the color.Color implementation of type they are converting to. This contradicts to the code of (*Surface).Set.
sdl/pixel.go:
```
func rgba8888Model(c color.Color) color.Color {
	if _, ok := c.(color.RGBA); ok {
		return c
	}
	r, g, b, a := c.RGBA()
	return RGBA8888{uint8(r), uint8(g), uint8(b), uint8(a)}
}
```