Hi!
I encountered a problem, if the underground belt in the graphics uses a mask, then the "sheet" is called "sheets".
I specified in the file "underground-heat-pipe.lua", line # 84:

local direction_out = assert(table.deepcopy(underground_belt.structure.direction_out.sheet)
  or table.deepcopy(underground_belt.structure.direction_out.sheets[1]))

The mod loads, but does not pull the mask, and the belts are lightless.
How to fix it, alas, I do not have enough knowledge :(

If you don't specify the first table, the mod gives an error with the arithmetic for "width".

include the name of the mod that adds masked belts.

This mod uses this code structure - https://mods.factorio.com/mod/AdvancedBeltsSA
File "core.lua", starting from line #296.

If you apply this function to standard underground belts, an error occurs.

core.set_structure_underground(data.raw["underground-belt"]["extreme-underground"], {color["extreme-add"], color["extreme-belt"]})

from file "data.lua", line #26.

For example:

core.set_structure_underground(data.raw["underground-belt"]["fast-underground-belt"], {color["extreme-belt"], color["extreme-belt"]})

It seems like it should be the color of the underground belt, but it turns out that it is colorless.

This bug is still not fixed :(

thanks for bumping, i had forgotten.

if i install both this mod and AdvancedBeltsSA then the express underground heat pipe is still blue, you must have some other mod installed too.

That's right, since in the standard settings the mod "AdvancedBeltsSA" does not affect the first 4 levels of underground belts.
As I wrote above, and if you try to apply the color replacement function for any of the first 4 levels of underground belts, an error appears :( That is, the mod changes the structure of the code, adding a layer level.
Can you take a look and try to make changes, or will you not bother?

oh it looked as though the color replacement you were mentioning required editing mod files directly, i only look to support it if its a startup option or set through a 3rd party compatibility mod.