Set transparent background of an imageview on Android

You can set the background transparent of any layout, any view, or any component by adding this code in XML:

android:background="@android:color/transparent" 

In your XML set the Background attribute to any colour, White(#FFFFFF) shade or Black(#000000) shade. If you want transparency, just put 80 before the actual hash code:

#80000000

This will change any colour you want to a transparent one.. :)

In addition to what Harshad mentioned:

Two hexadecimal characters can be appended to any hexadecimal color code. The first two characters in an 8-digit hex color code represents its opacity in Android.

The two hexadecimal characters can range from 00 to FF. For example,

• Normal opaque black hex- "#000000"
• Fully transparent - "#00000000"
• Fully opaque - "#FF000000"
• 50% transparent - "#7F000000"

This way you can change any color to any level of transparency.

To find the hexadecimal prefix from a percentage:

Divide the percentage number by 100 and multiply by 255 to get the decimal value. Convert the decimal to hexadecimal here.

For example, for 50%, 50/100 * 255 = 127. Using the link we get hexadecimal value 7F.

Source: Android: how to create a transparent or opaque background