How to change 'Pad Images' from black to white in Automator in Mac OS

HI There

I want to run an automation to scale mulitple images to 400 px square and have a WHITE background NOT black

have tried the most common answer and various versions of the below script:

Add in a 'run apple script' and changed the sizing to suit e.g 400 px

on run {input, parameters}  
repeat with this_file in input
    set this_path to the quoted form of the POSIX path of this_file
    do shell script "sips " & this_path & " -p 50 50 --padColor FFFFFF -i"
end repeat
return input
end run

nothing is working and I am m runing out of options and been through all the help forums online.

I've swapped the order of padding and apple script over too and that does not work

below is the current screenshot of what i have