#Pyth, 20 19 bytes

1 byte thanks to xnor.

%"#%06x"-t^8 8itz16

Try it online. Test suite.

###Explanation

• z is the input
• tz removes the #
• itz16 parses as a hexadecimal number
• t^8 8 calculates 8⁸ - 1
• -t^8 8itz16 calculates 8⁸ - 1 - input
• %"#%06x"-t^2 24itz16 formats it into a zero-padded 6-character hex string and adds the #

Pyth, 20 19 bytes

1 byte thanks to xnor.

%"#%06x"-t^8 8itz16

Try it online. Test suite.

Explanation

• z is the input
• tz removes the #
• itz16 parses as a hexadecimal number
• t^8 8 calculates 8⁸ - 1
• -t^8 8itz16 calculates 8⁸ - 1 - input
• %"#%06x"-t^2 24itz16 formats it into a zero-padded 6-character hex string and adds the #

#Pyth, 20 19 bytes

1 byte thanks to xnor.

%"#%06x"-t^8 8itz16

Try it online. Test suite.

###Explanation

• z is the input
• tz removes the #
• itz16 parses as a hexadecimal number
• t^8 8 calculates 8⁸ - 1
• -t^8 8itz16 calculates 8⁸ - 1 - input
• %"#%06x"-t^2 24itz16 formats it into a zero-padded 6-character hex string and adds the #

#Pyth, 20 19 bytes

1 byte thanks to xnor.

%"#%06x"-t^8 8itz16

Try it online. Test suite.

###Explanation

• z is the input
• tz removes the #
• itz16 parses as a hexadecimal number
• t^8 8 calculates 8⁸ - 1
• -t^8 8itz16 calculates 8⁸ - 1 - input
• %"#%06x"-t^2 24itz16 formats it into a zero-padded 6-character hex string and adds the #

#Pyth, 20 bytes

%"#%06x"-t^2 24itz16

Try it online. Test suite.

###Explanation

• z is the input
• tz removes the #
• itz16 parses as a hexadecimal number
• t^2 24 calculates 2²⁴ - 1
• -t^2 24itz16 calculates 2²⁴ - 1 - input
• %"#%06x"-t^2 24itz16 formats it into a zero-padded 6-character hex string and adds the #

#Pyth, 20 19 bytes

1 byte thanks to xnor.

%"#%06x"-t^8 8itz16

Try it online. Test suite.

###Explanation

• z is the input
• tz removes the #
• itz16 parses as a hexadecimal number
• t^8 8 calculates 8⁸ - 1
• -t^8 8itz16 calculates 8⁸ - 1 - input
• %"#%06x"-t^2 24itz16 formats it into a zero-padded 6-character hex string and adds the #