<?xml version="1.0" encoding="utf-8"?>
<resources>
    <color name="color_name">hex_color</color>
</resources>

<?xml version="1.0" encoding="utf-8"?>
<resources>
   <color name="opaque_red">#f00</color>
   <color name="translucent_red">#80ff0000</color>
</resources>

<TextView
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
android:textColor="@color/translucent_red"
    android:text="Hello"/>

Color State List Resource

A ColorStateList is an object you can define in XML that you can apply as a color, but will actually change colors, depending on the state of the View object to which it is applied.

For example, a Button widget can exist in one of several different states (pressed, focused, or neither) and, using a color state list, you can provide a different color during each state.

You can describe the state list in an XML file. Each color is defined in an element inside a single element. Each uses various attributes to describe the state.During each state change, the state list is traversed top to bottom and the first item that matches the current state will be used.

SYNTAX:

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android" >
    <item
        android:color="hex_color"
        android:state_pressed=["true" | "false"]
        android:state_focused=["true" | "false"]
        android:state_selected=["true" | "false"]
        android:state_checkable=["true" | "false"]
        android:state_checked=["true" | "false"]
        android:state_enabled=["true" | "false"]
        android:state_window_focused=["true" | "false"] />
</selector>

<selector>

This must be the root element. Contains one or more <item> elements.

attributes:
xmlns:android
Defines the XML namespace, which must be "http://schemas.android.com/apk/res/android".

<item>

Defines a color to use during certain states, as described by its attributes. Must be a child of a <selector> element.

Attribute	Description
android:color	Hexadeximal color.The value always begins with a pound (#) character and then followed by the Alpha-Red-Green-Blue information in one of the following formats: #RGB #ARGB #RRGGBB #AARRGGBB
android:state_pressed	Boolean. "true" if this item should be used when the object is pressed (such as when a button is touched/clicked); "false" if this item should be used in the default, non-pressed state.
android:state_focused	Boolean. "true" if this item should be used when the object is focused (such as when a button is highlighted using the trackball/d-pad);"false" if this item should be used in the default, non-focused state.
android:state_selected	Boolean. "true" if this item should be used when the object is selected (such as when a tab is opened); "false" if this item should be used when the object is not selected.
android:state_checkable	Boolean. "true" if this item should be used when the object is checkable; "false" if this item should be used when the object is not checkable. (Only useful if the object can transition between a checkable and non-checkable widget.)
android:state_checked	Boolean. "true" if this item should be used when the object is checked; "false" if it should be used when the object is un-checked.
android:state_enabled	Boolean. "true" if this item should be used when the object is enabled (capable of receiving touch/click events); "false" if it should be used when the object is disabled.
android:state_window_focused	Boolean. "true" if this item should be used when the application window has focus (the application is in the foreground), "false" if this item should be used when the application window does not have focus (for example, if the notification shade is pulled down or a dialog appears).

EXAMPLE:
XML file saved at res/color/button_text.xml:

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:state_pressed="true"
          android:color="#ffff0000"/> <!-- pressed -->
    <item android:state_focused="true"
          android:color="#ff0000ff"/> <!-- focused -->
    <item android:color="#ff000000"/> <!-- default -->
</selector>

This layout XML will apply the color list to a View:

<Button
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="@string/button_text"
    android:textColor="@color/button_text" />