Heroes 5 – by Azmadon (Izmail).

Yes, in the editor, I noticed that you can get a "divine white" color using Overlay.
Then, question #2: can I simply create a new texture or use other existing ones?

*Am I correct in understanding that the same scheme is used for "symbols," i.e., the "skirts" of units? And their color is adjusted similarly? For example, if I want to change the lower illumination of creatures and the hero on the map for the Horde, which files should I be looking at? 🤔

You need to change textures in the map editor because they appear as a whole list there. Spark is a spark, glow is a glow, etc. Although you can't preview them before selecting, at least by opening the hidden elements, you can see some kind of average color.

You can create a new texture, but it's a bit tedious. When you open an effect texture from Heroes, there's nothing on it – you need to save it without an alpha channel; then, the texture will have a black background. To remove it, you need to save the texture in TGA format and load it through the map editor with some settings (I don't remember which ones). If you decide to experiment, you need to resave the textures after the first import, specifically the TGA file – otherwise, you won't be able to load them back into the map editor.

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skirts are models.

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the textures for the skirts are located in \_(Model)\Creatures\Selection