Create spinner with strings taken from resource folder(res >> value folder). On changing spinner value, change background of screen.

first create resource file, right click on values directory>New>Values Resource file and name it array, and your array.xml file will created on values folder. Now add following code in array.xml

array.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <string-array name="S">
        <item>RED</item>
        <item>BLUE</item>
        <item>GREEN</item>
        <item>YELLOW</item>
    </string-array>
</resources>

activity_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity"
    android:id="@+id/rlVar1"
    >

    <Spinner
        android:layout_centerInParent="true"
        android:id="@+id/spinner1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:entries="@array/S"
        />
</RelativeLayout>

MainActivity.java

package com.example.helloworld;

import androidx.appcompat.app.AppCompatActivity;

import android.content.Intent;
import android.graphics.Color;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.Button;
import android.widget.EditText;
import android.widget.RelativeLayout;
import android.widget.Spinner;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Spinner spinner = findViewById(R.id.spinner1);
        RelativeLayout rl = findViewById(R.id.rlVar1);

        spinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
            @Override
            public void onItemSelected(AdapterView<?> adapterView, View view, int i, long l) {
                Toast.makeText(getApplicationContext(),adapterView.getItemAtPosition(i).toString(), Toast.LENGTH_SHORT).show();

                try {
                    rl.setBackgroundColor(Color.parseColor(adapterView.getItemAtPosition(i).toString()));
                }catch (Exception e)
                {
                    e.printStackTrace();
                }
            }

            @Override
            public void onNothingSelected(AdapterView<?> adapterView) {

            }
        });
    }
}

Output