TL;DR: I hate picking one. But if I had to, it’d probably be Black Car by Beach House.

Link to the Spotify Playlist. Depends what you define as alternative! Here’s what I found in my Spotify likes that one could argue are alternative (I’ve starred are my top faves):

⭐ Black - Pearl Jam (please listen to the MTV unplugged version if you haven’t yet)
Blackest Bile - Giles Corey
⭐ Blackberry Stone - Laura Marley
Black Swan - Thom Yorke
⭐ Black Car - Beach House
Black Cadillacs - Modest Mouse
Blackout - Muse
Black Rain - Soundgarden
⭐ Black Star - Radiohead
Waving A White Flag - The Smile
White Cyclosa - Boards of Canada
Red Right Hand - Arctic Monkeys
Ruby - Tweaker, Will Oldham (a bit of a weird one)
⭐ Ruby Pool - VOLA (alternative rock electronic stuff what genre even is this???)
⭐ Cherry-coloured Funk - Cocteau Twins
Cherry - Chromatics
Pink Funeral - Beach House
Pink Moon - Nick Drake
⭐ Peach Pit - Peach Pit
⭐ Tangerine - Frank Sativa
Amber - 311
Gold Guns Girls - Metric
I Love You Golden Blue - Sonic Youth
⭐ Green Green Grass Of Tunnel - múm
⭐ Turquoise Hexagon Sun - Boards of Canada

Some stuff I can’t say is alternative but I love enough to mention:

Big Black Car - Gregory Alan Isakov (Folk)
Black Leaf - John Paul White (that’s a twofer! Folk)
Black Sands - Bonobo (downtempo electronic? I dunno)
Red Summer - Thornhill (metalcore)
Vermilion, pt. 2 - Slipknot (this is acoustic, not what you might think as slipknot)
Flamingo - Kero Kero Bonito (don’t judge)
Bright as Yellow - The Innocence Mission (indie folk)
Aquamarine - The Afterimage (metalcore, this song very much not metal though)
Blue Ridge Mountains - Fleet Foxes (indie folk)