Color differences between 2D and 3D rendering

Hello, while working on a project I encountered a color difference between a 3D item and a 2D item. The 3D is rendered with a transparent background and the 3D item has no shading, so both 2D and 3D should have the same rendering. (I also disabled tone mapping but didn't change anything).

Please find attached a small example showing the problem, the test is with 2 colors : a kind of light blue and classic red.
3D cubes should match the color of rectangles above. The test use opacity 1 and 0.5, and for the 0.5 opacity : SrcAlpha and ConstantAlpha for blending.

The problem doesn't affect each color as the exemple can show. Also the way blending is set on custom material affect the rendering (I don't exactly understand what is the difference between ConstantAlpha and SrcAlpha / DstAlpha ?).

I don't know if it is a bug or if I missing something.

Best regards.