Changing the Imageview source from another java class

Question

In my application I am using the android fingerprint manager to verify the user, once the user has been verified, I would like the icon that is on the activity to another icon that is a different colour (same vector - different colour, different drawable file). Below is the code for the activity containing the imageview:

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

    keyguardManager = (KeyguardManager) getSystemService(KEYGUARD_SERVICE);
    fingerprintManager = (FingerprintManager) getSystemService(FINGERPRINT_SERVICE);

    if (!keyguardManager.isKeyguardSecure()) {

        Toast.makeText(this,
                "Lock screen security not enabled in Settings",
                Toast.LENGTH_LONG).show();
        return;
    }

    if (ActivityCompat.checkSelfPermission(this,
            Manifest.permission.USE_FINGERPRINT) !=
            PackageManager.PERMISSION_GRANTED) {
        Toast.makeText(this,
                "Fingerprint authentication permission not enabled",
                Toast.LENGTH_LONG).show();
        return;
    }

    if (!fingerprintManager.hasEnrolledFingerprints()) {

        // This happens when no fingerprints are registered.
        Toast.makeText(this,
                "Register at least one fingerprint in Settings to continue",
                Toast.LENGTH_LONG).show();
        return;
    }
    generateKey();
    if (cipherInit()) {
        cryptoObject = new FingerprintManager.CryptoObject(cipher);
        FingerprintHandler helper = new FingerprintHandler(this);
        helper.startAuth(fingerprintManager, cryptoObject);

    }
}

public boolean cipherInit() {
    try {
        cipher = Cipher.getInstance(
                KeyProperties.KEY_ALGORITHM_AES + "/"
                        + KeyProperties.BLOCK_MODE_CBC + "/"
                        + KeyProperties.ENCRYPTION_PADDING_PKCS7);
    } catch (NoSuchAlgorithmException |
            NoSuchPaddingException e) {
        throw new RuntimeException("Failed to get Cipher", e);
    }

    try {
        keyStore.load(null);
        SecretKey key = (SecretKey) keyStore.getKey(KEY_NAME,
                null);
        cipher.init(Cipher.ENCRYPT_MODE, key);
        return true;
    } catch (KeyPermanentlyInvalidatedException e) {
        return false;
    } catch (KeyStoreException | CertificateException
            | UnrecoverableKeyException | IOException
            | NoSuchAlgorithmException | InvalidKeyException e) {
        throw new RuntimeException("Failed to init Cipher", e);
    }
}
protected void generateKey() {
    try {
        keyStore = KeyStore.getInstance("AndroidKeyStore");
    } catch (Exception e) {
        e.printStackTrace();
    }

    try {
        keyGenerator = KeyGenerator.getInstance(
                KeyProperties.KEY_ALGORITHM_AES,
                "AndroidKeyStore");
    } catch (NoSuchAlgorithmException |
            NoSuchProviderException e) {
        throw new RuntimeException(
                "Failed to get KeyGenerator instance", e);
    }
    try {
        keyStore.load(null);
        keyGenerator.init(new
                KeyGenParameterSpec.Builder(KEY_NAME,
                KeyProperties.PURPOSE_ENCRYPT |
                        KeyProperties.PURPOSE_DECRYPT)
                .setBlockModes(KeyProperties.BLOCK_MODE_CBC)
                .setUserAuthenticationRequired(true)
                .setEncryptionPaddings(
                        KeyProperties.ENCRYPTION_PADDING_PKCS7)
                .build());
        keyGenerator.generateKey();
    } catch (NoSuchAlgorithmException |
            InvalidAlgorithmParameterException
            | CertificateException | IOException e) {
        throw new RuntimeException(e);
    }

Here is the XML for the same activity:

<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="@drawable/splashgradient"
tools:context="com.example.maxrw.tferfilemanager.HomeActivity">

<TextView
    android:id="@+id/textViewfinger"
    android:layout_width="309dp"
    android:layout_height="155dp"
    android:text="Please authorise using Nexus Inprint"
    android:textAlignment="center"
    android:textSize="40dp"
    android:layout_marginTop="48dp"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintTop_toTopOf="parent"
    app:layout_constraintRight_toRightOf="parent" />

<ImageView
    android:backgroundTint="@color/colorAccent"
    android:id="@+id/imageViewFinger"
    android:layout_width="196dp"
    android:layout_height="167dp"
    app:srcCompat="?android:attr/fingerprintAuthDrawable"
    android:layout_marginTop="68dp"
    app:layout_constraintTop_toBottomOf="@+id/textViewfinger"
    android:layout_marginRight="8dp"
    app:layout_constraintRight_toRightOf="parent"
    android:layout_marginLeft="8dp"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintHorizontal_bias="0.497"
    android:layout_marginStart="8dp"
    android:layout_marginEnd="8dp" />

And finally this is the code for the java class that links to the previously mentioned activity (this class has no activity of its own):

public class FingerprintHandler extends
        FingerprintManager.AuthenticationCallback {

    private CancellationSignal cancellationSignal;
    private Context appContext;

    public FingerprintHandler(Context context) {
        appContext = context;
    }

    public void startAuth(FingerprintManager manager,
                          FingerprintManager.CryptoObject cryptoObject) {

        cancellationSignal = new CancellationSignal();

        if (ActivityCompat.checkSelfPermission(appContext,
                Manifest.permission.USE_FINGERPRINT) !=
                PackageManager.PERMISSION_GRANTED) {
            return;
        }
        manager.authenticate(cryptoObject, cancellationSignal, 0, this, null);
    }

    @Override
    public void onAuthenticationError(int errMsgId,
                                      CharSequence errString) {
        Toast.makeText(appContext,
                "Authentication error\n" + errString,
                Toast.LENGTH_SHORT).show();
    }

    @Override
    public void onAuthenticationHelp(int helpMsgId,
                                     CharSequence helpString) {
        Toast.makeText(appContext,
                "Authentication help\n" + helpString,
                Toast.LENGTH_SHORT).show();
    }

    @Override
    public void onAuthenticationFailed() {
        Toast.makeText(appContext,
                "Authentication failed.",
                Toast.LENGTH_SHORT).show();
    }

    @Override
    public void onAuthenticationSucceeded(
            FingerprintManager.AuthenticationResult result) {

        appContext.startActivity(new Intent(appContext, MainActivity.class));

        imageViewFinger.setImageResource(R.drawable.fingerprintgreen);



        Toast.makeText(appContext,
                "Authentication succeeded.",
                Toast.LENGTH_LONG).show();

    }
}

The code that has the problem is at the end of the second java class:

imageViewFinger.setImageResource(R.drawable.fingerprintgreen);

Cheers for any help you guys can provide :)