How to change the color of the button you pressed?

I'm developing an app that randomly generates an add-in account and generates 4 values as possible responses and which are placed on 4 buttons (also in a random way).

I would like to know how I do it, when I click on any of the buttons (the possible answer), only it change color and do not let select others (do not let others change color).

Note: This process of choice would be repeated another 9 times.

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_realizar_contas);
        final TextView num1 = (TextView) findViewById(R.id.num1);
        final TextView num2 = (TextView) findViewById(R.id.num2);
        final Button op1 = (Button) findViewById(R.id.op1);
        final Button op2 = (Button) findViewById(R.id.op2);
        final Button op3 = (Button) findViewById(R.id.op3);
        final Button op4 = (Button) findViewById(R.id.op4);
        final Button avancar = (Button) findViewById(R.id.btn_avancar);

        esc = numeroDecisao();
        res1 = numeroAleatorio();
        res2 = numeroAleatorio();

        //Toast.makeText(getApplicationContext(), "Esc " + esc, Toast.LENGTH_SHORT).show();

        if (esc == 1) {
            val1 = res1 + res2;
            val2 = (numeroAleatorio() + numeroAleatorio()) + 1;
            val3 = (numeroAleatorio() + numeroAleatorio()) + 2;
            val4 = (numeroAleatorio() + numeroAleatorio()) + 3;

        } else if (esc == 2) {
            val1 = (numeroAleatorio() + numeroAleatorio()) + 3;
            val2 = res1 + res2;
            val3 = (numeroAleatorio() + numeroAleatorio()) + 7;
            val4 = (numeroAleatorio() + numeroAleatorio()) + 1;

        } else if (esc == 3) {
            val1 = (numeroAleatorio() + numeroAleatorio()) + 8;
            val2 = (numeroAleatorio() + numeroAleatorio()) + 9;
            val3 = res1 + res2;
            val4 = (numeroAleatorio() + numeroAleatorio()) + 10;
        } else if (esc == 4) {
            val1 = (numeroAleatorio() + numeroAleatorio()) + 7;
            val2 = (numeroAleatorio() + numeroAleatorio()) + 6;
            val3 = (numeroAleatorio() + numeroAleatorio()) + 5;
            val4 = res1 + res2;
        }

        num1.setText(Integer.toString(res1));
        num2.setText(Integer.toString(res2));
        op1.setText(Integer.toString(val1));
        op2.setText(Integer.toString(val2));
        op3.setText(Integer.toString(val3));
        op4.setText(Integer.toString(val4));

        op1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                op1.setBackgroundResource(R.drawable.botao_customizado);
                press = press + 1;
            }
        });


        //BOTÃO - AVANÇAR - Realizar a procdimento anterior 9 vezes
        avancar.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                int cont = 0;
                //Toast.makeText(getApplicationContext(), "Acertos " + acertos, Toast.LENGTH_LONG).show();

                while (cont != 9) {
                    res1 = numeroAleatorio();
                    res2 = numeroAleatorio();
                    esc = numeroDecisao();

                    if (esc == 1) {
                        val1 = res1 + res2;
                        val2 = (numeroAleatorio() + numeroAleatorio()) + 9;
                        val3 = (numeroAleatorio() + numeroAleatorio()) + 3;
                        val4 = (numeroAleatorio() + numeroAleatorio()) + 1;

                    } else if (esc == 2) {
                        val1 = (numeroAleatorio() + numeroAleatorio()) + 4;
                        val2 = res1 + res2;
                        val3 = (numeroAleatorio() + numeroAleatorio()) + 5;
                        val4 = (numeroAleatorio() + numeroAleatorio()) + 7;

                    } else if (esc == 3) {
                        val1 = (numeroAleatorio() + numeroAleatorio()) + 8;
                        val2 = (numeroAleatorio() + numeroAleatorio()) + 9;
                        val3 = res1 + res2;
                        val4 = (numeroAleatorio() + numeroAleatorio()) + 10;
                    } else if (esc == 4) {
                        val1 = (numeroAleatorio() + numeroAleatorio()) + 10;
                        val2 = (numeroAleatorio() + numeroAleatorio()) + 3;
                        val3 = (numeroAleatorio() + numeroAleatorio()) + 1;
                        val4 = res1 + res2;
                    }

                    num1.setText(Integer.toString(res1));
                    num2.setText(Integer.toString(res2));
                    op1.setText(Integer.toString(val1));
                    op2.setText(Integer.toString(val2));
                    op3.setText(Integer.toString(val3));
                    op4.setText(Integer.toString(val4));
                    cont++;


                }
            }
        });
    }