The foreign regiments were officially designated as light infantry, and so were uniformed as such.

The Otto Manuscript has a voltigeur officer of the 24th Ligne in a colpack, so it was not unknown in line infantry units, at least for officers. Line drum majors were sometimes uniformed with colpacks, again depending on the regimental commander.

So, the 'answer' is that some line infantry voltigeurs had colpacks like their light infantry brethren.