rainbow textview android ref: http://chiuki.github.io/android-shaders-filters/#/5

RainbowTextView.java

public class RainbowTextView extends TextView {
  protected void onSizeChanged(int w, int h, int oldw, int oldh) {
    super.onSizeChanged(w, h, oldw, oldh);

    int[] rainbow = getRainbowColors();
    Shader shader = new LinearGradient(0, 0, 0, w, rainbow, 
        null, Shader.TileMode.MIRROR);

    Matrix matrix = new Matrix();
    matrix.setRotate(90);
    shader.setLocalMatrix(matrix);

    getPaint().setShader(shader);
  }
  
  private int[] getRainbowColors() {
    return new int[] {
      getResources().getColor(R.color.rainbow_red),
      getResources().getColor(R.color.rainbow_yellow),
      getResources().getColor(R.color.rainbow_green),
      getResources().getColor(R.color.rainbow_blue),
      getResources().getColor(R.color.rainbow_purple)
    };
  }
}