I think the best analogy isn't something like "Black to White to Technicolour" - that sounds far too immediate and dramatic and vivid. The other thing is that parenting a child is very different depending on what stage you're at. A newborn is one thing, an older baby another, a toddler another, a pre-schooler another and school child another and I haven't yet got beyond that to the dreaded teens. I don't think it gets "easier", just "different challenges' as they grow. Small babies needs are very urgent and you don't get much sleep. But they don't tantrum, they don't argue, they don't demand crazy things. A three year old probably sleeps OK but you have the whole strong will thing going on and the rages if they don't get their own way. O
Its a very gradual change, not a technicolour . A lot of it is a slog. You really have to grow up, not only to take the responsibility but also to be the adult and not descend to the child's level, to be patient and forbearing and not just "lose it" like the child is doing. You have to do this even if you are totally knackered and you have a big deadline looming at work so that even when the little sausage is in bed you then have to log on and work into the night. You have to think about their needs all the time and your needs get totally lost in their needs, esp when they are little. When they get to school you can start to reclaim some of your own life. There wouldn't be much sitting around in bed reading and watching TV though, maybe for an hour in the evening when they are in bed!
That said, they bring so much joy and so much purpose to life. Life becomes more mundane but often it is easier to feel content. Less striving, less soul searching. More just getting on with things and muddling along.
It totally wrecks your coupledom life, you're under so much pressure and so knackered. Roles get polarised. But on the other hand, you can work through it and come out closer and find a way to stay close despite the bomb that has gone off in your life! And there is a good deal of shared joy in your mutual offspring!
I wouldn't think too much. You're thinking a lot - its not like buying car A or car B. Just ask yourself, do I feel the urge or not? If I get to 70 and I have no kids, will i be sad?
If you feel the urge then go for it and don't worry about the rest. It will all work out.
It may be a total challenge but then, if you choose not to have kids, you'll stay stuck where you are now. If you have them, you'll have to grow personally. And personal growth is life giving, even if its not always easy.