This is super helpful - I didn't really understand that the color-coding system affected the effectiveness of reel. I thought the gauge being red was more of a "high risk/high reward" situation - your line has a chance of breaking, but the tradeoff is that you can reel in faster. I'll try going green next time.Ideally you don't want that semicircular gauge to max out but you can upgrade that to a lot bigger as you find and combine/upgrade other rods. Yeah you just have to let it off keep an eye on the fish icon when it's green get reeling but when it's yellow or red hold off a bit as trying to reel in when red isn't effective and stresses out the rod and line gauge a lot faster.
If you get the green circle gauge on the top right then you have caught a boss fish, you need to get the entire circle gauge filled in red however there is a time limit, when the gauge is full you then have to reel it in. You still have to be careful of the other gauge that you don't break your line. Depending on your rod stats will determine how fast you can get the gauge to fill however if you fail and lose the fish the circle gauge will remember where you got to so when you get the boss fish again it will continue from where you last filled the gauge to making it easier to get the boss fish.
I'll keep an eye out for the boss fish gauge too (something else I didn't know about). Thanks again!