You can add particle effects that look like fire easily, see particles, but if you actually want to set Goo Balls on fire, you need a fire object.
Fire objects are circular so have a center and a radius. Any Goo Balls with a non-zero burntime attribute are flammable*, and will catch fire if they enter a fire object.
Note: Thrown balls do not catch fire, unless they bounce off something first.
You can control what the fire looks like using the particles attribute. You can select any local particle effect, although for playability you should choose
- an effect that looks like fire
- an effect that approximately matches the size of the fire object.
* Balls that are flammable
- Fuse
- Ivy
- Bit, Pixel, PixelProduct
- BombMini, BombShaft, BombSticky, ZBomb
- Drained, DrainedIsh
- BeautyProduct (but not BeautyProductEye, or UglyProduct)
Ivy balls are "flammable" in that the same "fire" property is used for "poisoned", just with different particle effects. Therefore it is inadvisable to use Ivy balls on levels containing real fire.