50% opacity at white background for textview

In your layout file just set the background of the TextView to "#8FFF"

<TextView
android:layout_width="..."
android:layout_height="..."
android:background="#8FFF"
/>

Here 8 is the alpha value, FFF are the RGB values respectivley. See here for more info on the background attribute

In the tutorial that you mention, you will notice that the color value has two digits more that what you are specifying. You specify #FFFFFF (6 digits), whereas the tutorial specifies #CCFF0000 (8 digits). The two first digits that were add (CC) represent the alpha.

so in your case try something like #AAFFFFFF