vertex paint Do not work in ue4.18

Hello!

hey guys, am I wrong or vertex paint do not work on unreal4.18?
I did follow a tutorial and I am sure the material is set up properly, But I can’t really paint anything with alpha option ticked. Shift pressed or not I can’t see anything getting painted.

I am just trying to paint some ground texture and some moss. However if I switch alpha off and I tick one of the RGB boxes I see some color getting painted but is not what I want.

Anyone can help?
Any reason for that?

Best

Giulio