I want to use a logoTexture that has a built-in red background, so the loading screen background can still be white, as well as lifting it up (vertically) a bit.
What I've tried:

• Using Resource Loader to override the existing texture (with the default path under the modernsplash namespace), but then it just shows the default texture the mod ships with.
• Using Resource Loader to set a new resource location under the same or a different namespace and target it in splash.properties, but it shows the whole loading screen as black and then crashes - it seems to not recognize the new texture (or any new namespace).

The only solution that worked was manually changing the file inside modernsplash-1.12.2-1.2.2.2.jar.

The splash screen is at very low level and is using .minecraft/resources folder as resource pack.
Using a resource loader to do this may produce conflict, since they are using the same path

The default namespace is in fact modernsplash:textures/gui/title/mojang.png
Just put it in .minecraft\resources\modernsplash\textures\gui\title\mojang.png should work

• Trying to use Resource Loader to override the texture in \resources\minecraft\textures\gui\title\mojang.png doesn't work (it just shows/prioritizes the default texture shipped with the mod)
• Trying to change the config to use logoTexture=minecraft\:textures/gui/title/mojang.png just shows the old (1.12.2 default) texture
• Trying to launch with a resource pack overriding either modernsplash\textures\gui\title\mojang.png or minecraft\textures\gui\title\mojang.png just shows/prioritizes the default texture shipped with the mod.

Unless I did something wrong, do you know any other way to modify the texture, or do you recommend just shipping the mod with the texture changed inside?
Another possible idea is having the built-in red background as a configurable option.

That's probably the first thing I tried and unfortunately it does not work (it shows the default texture shipped with the mod jar)

Sorry but I haven't made resource packs in a while, the correct path is .minecraft\resources\assets\modernsplash\textures\gui\title\mojang.png

Oh, right. I'm silly.
Thank you for the help.