#[MATL](https://github.com/lmendo/MATL/releases/tag/6.0.0), 21 bytes

    35,5Y216,j6L)!16ZA-)h

###Example

This has been executed on Octave:

    >> matl
     > 35,5Y216,j6L)!16ZA-)h
     >
    > #FFAA20
    #0055DF

###Explanation

    35,             % nunber literal: ASCII code of '#'
    5Y2             % '0123456789ABCDEF'
    16,             % number literal
    j               % input string
    6L)             % remove first element
    !               % transpose
    16ZA            % convert from hex to dec
    -               % subtract from 16
    )               % index into '0123456789ABCDEF' to convert back to hex
    h               % concatenate 35, which is '#'