Try this code :
Compare String using .equals
or .equalsIgnoreCase
method.
final String a="cout";
Button okButton = (Button) findViewById(R.id.OK);
okButton.setOnClickListener(new View.OnClickListener(){
@Override
public void onClick(View view){
EditText text = (EditText)findViewById(R.id.Vprasanje1);
String value = text.getText().toString();
if(value.equals(a))
{
EditText banana = (EditText) findViewById(R.id.Vprasanje1);
banana.setTextColor(Color.GREEN);
}