How did the colors switch from vibrant yellows and greens to dull blacks and blues?

Each chapter has 10 memes Slight anime spoilers, no manga spoilers, almost none of these memes are mine and go to the proper owner, all memes I found with credit in photo I did not crop out so all that had credit when I found them still have it, if anyone made art/memes and you found it in here, please dm and I'll take it down for you, same goes if you want me to post a meme, dm me and it will probably get posted here^^

William Magpie is a researcher working on one of the world's largest project in the history of mankind. A mutagen in many of the contained but not cured diseases and disorders has caused them to go on a rampage throughout the globe. He, and many other people from across the seven seas have been tasked with trying to finally find a cure for this global disaster. But with great responsibility comes great danger. Lets see how dark this can get.

yup... so this is my first book so I hope it doesn't suck. Its legit just a bunch of memes. No judgement. if you want some thing then just request and I'll try to find it. A lot of these are from other stories on wattpad so if you want me to take it down just say so. any ways I'm done. oh wait ..... I DID NOT CREATE ANY OF THE MEMES! I DO NOT TAKE CREDIT FOR THE MEMES, ONLY THE BOOK!