Girls Mode 3 introduces a reason to take pictures in game: the color palette. You start off with only a few colors in each shade, but when you go to different locations and take pictures you can show the pictures to a girl in the park. If you’re lucky she’ll find a new color in the photo and it will be added to your color palette. You can then use those colors for hair, makeup (and presumably clothes design, but I haven’t unlocked clothes design yet).
Either you can run around and take pictures of everything hoping there may be a color in the photo (did that), or you can chat with people who have rainbow colored exclamation marks next to them for a hint about a photo to take. Here are the color palette colors with the photos I used to unlock them. I’m not sure if it’s more helpful to be in palette order, or location order, but I’m going with palette order for these.
Since the game is in real time, I’m missing a few seasonal colors and I haven’t unlocked a couple areas or seen a few cutscenes for colors. I’ll add in those pictures as I unlock them. There may be duplicate images because some images give you more than one color.
Also note, these are direct translations of the Japanese names and the English names may change when (crossing fingers) the game gets localized.
Black/White
Sugar White- Default Color
Snow White- Flower Garden
At the flower bed during the winter in the morning and afternoon. 
Light Gray- Default Color
Station Gray- Station Plaza
View of the station in the morning or afternoon.
Cat Gray- Midori Park
Take a picture of the grey cat on the park bench in the morning or afternoon
Live Ash- Live House (the club)
Talking with an NPC (I got this in the evening)
Rock Slate Gray- Live House (the club)
I got this in the afternoon while having an air guitar session.
Black- Default Color
Blue
Cycle Baby Blue- My Road
Take a picture of the bicycle in the morning or afternoon.
- Lake Blue
- Midori Park
I think this must be taken during the day.
Madonna Blue- Photo Studio
Inside the photo studio.
Facade Blue- Station Plaza
The blue front of the photo studio in the station plaza.
Soda Blue- Default Color
Blue on the Stage- Candyland (the amusement park)
Take a picture of the people singing onstage.
Blue Hawaii- Default Color
Dark Ocean- Tokonuts Beach
Take a picture on the beach at night.
Stardom Blue- Arena
Take a picture talking to people in front of the arena.
Romance Night View- Restaurant
The skyline in the restaurant at night.
Blue-Purple
Illuminate Purple- Dress-Up Square
The buildings in dress-up square at night.
Crocus- Default Color
Salvia- Flower Garden
In front of the mansion.
Metro Police Navy- The City
The building in the background lit up at night.
Rabbit Purple- Candyland (the amusement park)
In front of the rabbit signs
Violet- Default Color
Starry Sky- Tokonuts Beach
At the bell promontory at night.
Sky Night- Tokonuts Beach
The view of the night sky from the beach.
Midnight Marine- The City
Two colors from the crepe gobbling picture.
Night Park- Midori Park
Night overview of the park.
Purple
Red Bean Milk- Default Color
Station Bricks- Station Plaza
The bricks in front of the station entrance.
Ristorante- Restaurant
Take a picture sitting at the table in the restaurant.
Amethyst- Candyland (the amusement park)
Take a picture riding the ferris wheel at night.
Lilac- Default Color
Rock Violet- Café Street
Take a picture of the post to the right of the stairs to the club.
- Berry Berry Cake
- Café Street
Speak with the 2nd NPC from the left at café street until you trigger the berry berry cake event.
Pansy- Midori Park
In front of the pansy flower bed from the Spring through Fall, in the Morning to the Evening.
- Grape
- Tokonuts Beach
Speak with Luin in front of the Terrace Café to trigger the fruit platter event.
Dark Purple- Flower Garden
Flower Garden at night.
Pink
Cosmos- Default Color
Flower Bouquet- Flower Garden
During the day at the flower garden.
Beauty Pink- Makeup Salon
First Floor
Lantern- Asian Pathway
Must be taken at night when the lanterns are lit up.
Girls Pink- Default Color
Bougainvillea- Tokonuts Beach
Terrace café in the morning-evening with a view of the bougainvillea flowers.
Strawberry Pink- Restaurant
Snap a picture at the restaurant with the strawberry cake on the table.
Afternoon Rose- Rose Garden in Midori Park
doesn’t actually have to be taken in the afternoon, just from the spring to the fall
City- City
Morning and Evening
Cherry Blossom- Asian Pathway
At the cherry blossom tree only in the spring.
- Party Bouquet Pink
- Restaurant
Snap a picture of the big rose bouquet during a party.
Red
Tomato- Default Color
Hibiscus- Default Color
- Poinsettia
- Designer Lounge
Snap a picture in the designer lounge (see the poinsettias in the background?)
Ruby- Default Color
Dalia- Flower Garden
Flower Bed in front of the mansion in the morning or afternoon.
Darjeeling- Rose Garden in Midori Park
Having tea with an NPC
Garnet- Default Color
Trend Garnet- Exhibition Hall
As you gaze in amazement at the entrance.
Chocolate Cake- Café Street
Morning and Evening with cake on the café table. (I’m pretending it’s red velvet)
- Night Rose
- Midori Park
**Missing. The roses in the fall during the evening, or in the winter at night. 
Piano- Restaurant
Get your jamming on at the piano and then immortalize your shame.
Light Brown
Sand Beige- Tokonuts Beach
At Tokonuts Beach in the morning-evening.
Champagne- Default Color
Walnut- Default Color
Brick Tile- Rose Garden
Take a picture of the bricks in the rose garden.
Café- Café Street
Take a picture on café street with the café visible.
Milk Chocolate- Default Color
Maroon- Default Color
Tokonuts Café- Tokonuts Beach
The front of the Terrace Café at Tokonuts Beach.
Tea Tree- Café Street
The cherry tree by the live house in the morning-evening.
Elegant Rose- Restaurant
The rose on the piano (while belting out a tune)
- Tiramisu
- Café Street
**Missing Have a conversation with an NPC in front of the café that results in gorging on Tiramisu.
Dark Brown
Cocoa- Default Color
- Atelier Brownie
- My Road
The second floor of your clothing shop.
Seaside Terrace- Tokonuts Beach
The front of the Terrace Café at Tokonuts Beach.
Khaki- Default Color
Owlstone- Asian Pathway
Snap a picture with the owl statue.
Science Olive- Makeup Salon
Take a picture in the second floor of the makeup salon
Antique Door- Flower Garden
Snap a pic in front of the mansion
Bitter Chocolate- Default Color
European Sepia- Flower Garden
Snap an overview pic of the mansion in the flower garden.
More to come!
Thanks to mizunosakura for pointing me to the color guides: Girls Mode color palette guide and this google doc.
