Bases and alkali's turn litmus paper purple.
It will go a pink purple couler indicating that it is an alkali.
Litmus paper that turns purple indicates neutral pH.
Purple litmus (neutral) become red in acidic solutions.
In a neutral solution the litmus paper is purple.
In a neutral solution litmus has a purple color.
You can add the color blue to make purple more blue.
Neutral litmus is purple at pH=4,3.
Litmus is red at a pH under 4,5.
After the complete neutralization the litmus color remain unchanged - purple.
add red!
bule
I'm afraid you can't. Blue and red make purple.