Configure the option "Spell-checking color" (toggle between standard and user-defined; File | Options)

Here you can configure if Presentations paints the line under misspelled words in a user-defined color (value ON) or not (value OFF).

SpellCheckColorEnable

0. Off
   

   Registry Hive	HKEY_LOCAL_MACHINE or HKEY_CURRENT_USER
   Registry Path	SOFTWARE\Policies\SoftMaker\Presentations 2018
   Value Name	SpellCheckColorEnable
   Value Type	REG_DWORD
   Value	0

1. On
   

   Registry Hive	HKEY_LOCAL_MACHINE or HKEY_CURRENT_USER
   Registry Path	SOFTWARE\Policies\SoftMaker\Presentations 2018
   Value Name	SpellCheckColorEnable
   Value Type	REG_DWORD
   Value	1