From Xojo Documentation
The amount of transparency that affects all drawing to the Graphics object done afterwards. The range is from 0.0 (opaque) to 100.0 (fully transparent). Transparency of the Graphics object can be used to obtain “fading” effects (amongst other uses) by setting the transparency to a value greater than 0.0.
When rendering colors or pictures with alpha channel information, the Transparency property is composited with the alpha channel information. This allows you to perform uniform fading operations easily and efficiently.
On Windows, using the Transparency property only works on Graphics if the Picture was created with a depth of 32, or is a Picture with an alpha channel. Attempting to use it on other forms of Pictures will result in an UnsupportedOperationException.
Graphics transparency is supported on all platforms.
Create a picture with a blue rectangle drawn with 50% transparency: