The documentation for ColorOverlay contains the following strings:

In "Detailed description":

"The color for the overlay is given in the RGBA format"

In the "color" property documentation:

"This property defines the RGBA color value which is used to colorize the source."

This is incorrect. The color is ARGB, not RGBA. (See the examples on that page, or click through to the documentation for the color qml basic type)