Change gradient color step in expression

I want to change fill color of vector object in expression. The problem is the color is gradient in 3 step and I don’t know how to set new gradient color for each step by using expression language.
Is this possible?

Is this what you are looking for?

No this function mix two color

There’s no way to modify an actual gradient object with expressions. You’ll need to use scripting.

Can you guide how to do that?
I’m not Java programmer and I need example for that

Could you cut your vector graphic in half, group it, then use the gradient expression function on each half to get the desired effect? Not sure if that would be easier than the scripting method, but might be an option.