[simple question] custom expression

I think this is a very simple question for you.
I’m quite new to ue4.

I want to use custom expression nodes, I’m just playing with them and I see that I cannot generate a color from a custom expression node,
for example if inside the custom expression I do something like this:
float3 mycolor = (1.0,0.0,0.0); return mycolor;

and then I connect the output of the custom expression block to base color of a material I would expect a red color. Instead I have a black color…

Why? What I’m missing! I think that code is the simplest I can try.

Look also at this case:
The input is 0,0,0, I add 1,0,0… the result is black… why? should not be red again?

Anybody know how to make this work? :confused: