Change the Car's Colours

A paint job is the cheapest thrill in any car game. Let's add one — and learn the real, sometimes-restrictive truth about materials in Verse.

The honest truth about materials

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Material Swap

You might expect a SetColor or SetVectorParameter call. It doesn't exist for creators. In build 41.00 the material class is <epic_internal>: its SetColorProperty / SetFloatProperty methods are internal, and there is no public material_parameter_collection, dynamic_material, or per-parameter setter anywhere in the API. You cannot tweak a material's colour value from Verse.

What you can do is swap the whole material on a placed prop, with one public call:

creative_prop.SetMaterial(Material : material, ?Index : int)

So the recipe is: author a few material assets (a red paint, a blue paint, a gold paint) in your project, expose them as editable slots, and cycle through them on a button press. Same car body, instant new look.

Editable material slots: the ?material idiom

There's a wrinkle. Because material is internal, you can't write material{} to give a data member a default — the compiler rejects the constructor. The idiom is to declare the slot as an optional that defaults to false:

@editable
PaintA : ?material = false

The editor fills the slot with a real material asset; in code you unwrap it with PaintA?. (Try PaintA : material = material{} and you'll get "Invalid access of epic_internal class constructor" — proof of the rule.)

Cycling the paint

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Paint Cycle Swap

On each button press we advance an index 0 → 1 → 2 → 0 and apply whichever slot is filled:

OnPaintPressed(Agent : agent) : void =
    if (Next := Mod[PaintIndex + 1, 3]):
        set PaintIndex = Next
        if (PaintIndex = 0, Paint := PaintA?):
            ApplyPaint(Paint)
        else if (PaintIndex = 1, Paint := PaintB?):
            ApplyPaint(Paint)
        else if (Paint := PaintC?):
            ApplyPaint(Paint)

Two small Verse details earn their keep here. Mod[...] is a failable (<decides>) call, so we compute it inside an if and then set the variable — you can't put a failable expression on the right of a set. And each branch tests two things at once: the index matches and the optional unwraps (Paint := PaintA?), so an empty slot is simply skipped.

ApplyPaint does the actual swap on every tagged car body:

ApplyPaint(Paint : material) : void =
    for (Obj : FindCreativeObjectsWithTag(car_body_tag)):
        if (Body := creative_prop[Obj]):
            Body.SetMaterial(Paint, ?Index := 0)

Note ?Index := 0 — SetMaterial's index is a named optional parameter, so it's passed with the ?Name := value syntax, not positionally.

The full device

Compile-verified in UEFN 5.8. Tag your car body car_body_tag, a button car_paint_button_tag, drop three Material assets into the slots, and press to cycle.

Recap

• There is no public per-parameter colour API — material is internal; you swap whole materials.
• creative_prop.SetMaterial(Material, ?Index := 0) is the one public paint call; the index is a named optional.
• Declare asset slots as ?material = false (you can't construct material{}); unwrap with Paint?.
• Mod[...] is failable — compute it in an if, then set.