docs: add information about using color resources as drawables

  </div>

</div>

<p>A drawable resource is a general concept for a graphic that you

can retrieve with {@link android.content.res.Resources#getDrawable(int)}

and draw on the screen. There are several different types of drawables:</p>

<p>A drawable resource is a general concept for a graphic that can be drawn to the screen and which

you can retrieve with APIs such as {@link android.content.res.Resources#getDrawable(int)} or apply

to another XML resource with attributes such as {@code android:drawable} and {@code android:icon}.

There are several different types of drawables:</p>

<dl>

  <dt><a href="#Bitmap">Bitmap File</a><dt>

    <dd>A bitmap graphic file ({@code .png}, {@code .jpg}, or {@code .gif}).

<p>Also see the <a href="animation-resource.html">Animation Resource</a> document for how to

create an {@link android.graphics.drawable.AnimationDrawable}.</p>

<p class="note"><strong>Note:</strong> A <a

href="{@docRoot}guide/topics/resources/more-resources.html#Color">color resource</a> can also be

used as a drawable in XML. For example, when creating a <a href="#StateList">state list

drawable</a>, you can reference a color resource for the {@code android:drawable} attribute ({@code

android:drawable="@color/green"}).</p>

<h2 id="Color">Color</h2>

<p>A color value defined in XML.

The color is specified with an RGB value and alpha channel. You can use color resource

any place that accepts a hexadecimal color value.</p>

The color is specified with an RGB value and alpha channel. You can use a color resource

any place that accepts a hexadecimal color value. You can also use a color resource when a

drawable resource is expected in XML (for example, {@code android:drawable="@color/green"}).</p>

<p>The value always begins with a pound (#) character and then followed by the

Alpha-Red-Green-Blue information in one of the following formats:</p>