The entries in each half clue a celebrity, but not all are unambiguous enough to point to only one person.
The key is that each celebrity in the second half was born with a different surname than the one they are currently widely known by.
This birth name matches the current surname of a celebrity clued in the first half. The first group of clues is alphabetized by the celebrity’s commonly used first name. The second group is alphabetized by the celebrity’s first name at birth.
These are the pairings, in the order provided by the M&M photos:
|Celebrity 1||Celebrity 2||Birthname||Green M&Ms||Letter|
|Upton Sinclair||Vin Diesel||Mark Sinclair||2||I|
|George Harrison||Courtney Love||Courtney Harrison||7||O|
|Kevin McDonald||David Tennant||David McDonald||5||N|
|John Edwards||Shania Twain||Eilleen Edwards||1||E|
|Jim Kelly||Iggy Azalea||Amethyst Kelly||3||L|
|Keith Hernandez||Bruno Mars||Peter Hernandez||2||E|
|Philip Larkin||Joan Jett||Joan Larkin||5||I|
|Tammy Duckworth||Kendrick Lamar||Kendrick Duckworth||8||T|
|Tim Cook||Alicia Keys||Alicia Cook||1||C|
|Chuck Palahniuk||Jack Palance||Volodymyr Palahniuk||5||H|
|Jim Morrison||John Wayne||Marion Morrison||6||S|
|Shel Silverstein||Andrew Dice Clay||Andrew Silverstein||8||T|
|Alan Greenspan||Jason Alexander||Jay Greenspan||8||A|
|Fred Rogers||Portia de Rossi||Amanda Rogers||3||G|
|James Garfield||Henry Rollins||Henry Garfield||6||E|
|Don Johnson||Whoopi Goldberg||Caryn Johnson||4||N|
|Chris Pratt||Boris Karloff||William Pratt||3||A|
|Narendra Modi||Kal Penn||Kalpen Modi||1||M|
|Patty Duke||Sean Astin||Sean Duke||4||E|
The number of blue M&Ms is the enumeration of the first name of the celebrity in the first group, while the number of yellow M&Ms is the enumeration of the birth first name of the celebrity in the second group (with the M visible when an M occurs at that position).
Indexing into the shared surname by the number of green M&Ms gives the phrase IONE LEITCH STAGE NAME.
Ione Leitch is the birth name of Ione Skye, so the answer is IONE SKYE.