To me, the color means I am not sending/receiving images/video at full quality.

In my extended family, we even have both iMessage groups and WhatsApp groups, the iMessage ones specifically when we want to share original quality pics/videos of the kids and family for whoever wants to save them.

On Android you can send images as files in Whatsapp and most other messengers. Does Apple not allow such access to your own photo files?

It's not an Apple thing, it's a carrier thing. MMS (green) data sizes are controlled by the carrier. WhatsApp doesn't use MMS and is a closed protocol.

iMessage (blue) sent messages are similarly a closed protocol and unencumbered by the carrier.

The destination carrier determines the maximum allowable size an MMS attachment can be:

    Tier 1 Carriers -- Verizon, T-Mobile, AT&T -- all MMS content up to about 1 MB.  

    Tier 2 Carriers allow MMS content of 600 KB.  

    Tier 3 Carriers allow MMS content of 300 KB.

MMS (“green messages”) are mangled by most US carriers. The carrier will often recompress the media. And phones tend to send it at a lower quality to begin with.

I had never thought of doing that. I just tried, and clicking the + symbol in WhatsApp gives me option to send photos/video, documents, contact, and location.

If I choose documents, I do not see where I can go to select photos/videos in the iPhone camera roll. I presume it may not be accessible that way.

Also, I presume WhatsApp does not want to allow people to send photo/video at full size in their network to reduce bandwidth costs.

Exactly. When I see a green message bubble on my iPhone, it means that I'm in a gimped environment, especially if it's a group chat. I can't leave an SMS group chat unilaterally. I can't direct-reply to specific messages. Emoji-reactions to messages will render as "John liked 'Check out this website!'" Links won't get rich previews.

Personally I'd take that as a cue to switch to a more inclusive messaging platform but that's just me.

Easy for me to say I guess. There's never been enough iOS ubiquity in my circle for the network effect to take over. There's age, nationality and class factors at play. UK had slightly less iOS dominance than the US and I'm older.

> Links won't get rich previews.

as a counterpoint, iMessage messes up links sometimes, I had to turn it off because it breaks some types of links that I sometimes have to exchange with my coworkers. SMS is plain text, so the link is preserved as-is

Edit: those are links generated using Firebase Dynamic Links service. When sent over Twilio (vanilla SMS) they work but when forwarded from iPhone to iPhone they will break unless you turn off iMessage.