Subscriber only called when the validity changes. For this, we’d need to call distinctUntilChanged() method on our Observables.

MainActivity.java

        isUsernameValid.distinctUntilChanged().doOnNext(new Consumer<Boolean>() {
            @Override
            public void accept(Boolean valid) throws Exception {
                Log.d(TAG, "Username " + (valid ? "Valid" : "Invalid"));
            }
        }).map(new Function<Boolean, Integer>() {

            @Override
            public Integer apply(Boolean valid) throws Exception {
                return valid?Color.WHITE:Color.RED;
            }
        }).subscribe(new Consumer<Integer>() {

            @Override
            public void accept(Integer color) throws Exception {
                mUsernameEditText.setTextColor(color);
            }
        });

        isEmailValid.distinctUntilChanged().doOnNext(new Consumer<Boolean>() {
            @Override
            public void accept(Boolean valid) throws Exception {
                Log.d(TAG, "Email " + (valid ? "Valid" : "Invalid"));
            }
        }).map(new Function<Boolean, Integer>() {

            @Override
            public Integer apply(Boolean valid) throws Exception {
                return valid?Color.WHITE:Color.RED;
            }
        }).subscribe(new Consumer<Integer>() {

            @Override
            public void accept(Integer color) throws Exception {
                mEmailEditText.setTextColor(color);
            }
        });

        registerButtonObservable.distinctUntilChanged().subscribe(new Consumer<Boolean>() {
            @Override
            public void accept(Boolean aBoolean) throws Exception {
                mRegisterButton.setEnabled(aBoolean);
            }

        });