String texto = "Texto para usar";

    Spannable timeSpannable = new SpannableString(texto.toUpperCase());

    boton.setText(timeSpannable);

public static SpannableString highlight(String s, int k) {
    SpannableString ss = new SpannableString(s);
    ss.setSpan(new ForegroundColorSpan(Color.BLUE), 0, k, Spannable.SPAN_EXCLUSIVE_INCLUSIVE);


    return ss;
}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    LinearLayout.LayoutParams layoutparams = new LinearLayout.LayoutParams(
            LinearLayout.LayoutParams.MATCH_PARENT,
            LinearLayout.LayoutParams.WRAP_CONTENT);

    Button boton = (Button) findViewById(R.id.ir);

    boton.setText("Texto para usar");

    boton.setLayoutParams(layoutparams);

    boton.setText(highlight(boton.getText().toString(),3));
}