1. I am very sorry to dissapoint you with my answer, but we have to be realistic and it would not be good if someone will mislead you about the subject.
Unfortunately you can not change your eye color by a spell, and you can not be change anything with your body as well, only diet spells which give you motivation/make your body ill to lose weight work. A spell depends on the energy you send as well, and energy will never be able to change anything with you eyes, you should love your eyes with the way the look right now,. Even if you try this spell 1000 times, you will see no results it is just pointless and a loss of time. It is not good to be so happy about a thing which is not possible and then to discover you were wrong, it is better to be told about a mistake before you make the wrong thins, i just do not want people to mislead you. Eye color depends on your genes and other things, and a spell can not change you genes, that's why we have photoshop. Yoy might want to look for medical ways to do it, but i do not thing that there is something like this right now, but i hope there will be . And i really recommend you to love yourself in the way you are, not matter what color your eyes are. It will not change anything if your eyes are brown and not blue.
2. It depends on what you think and prefer. Basically, people say that writing your own spells is better because you have to remember that when you write your own spells, you put your own energy into them, and these will work better because of that. (This same technique can also be used for rewriting other spells that you may find). You also match it better to your intent, will and desire much better since you are the one who writes it. You are capable of being much more precise with you words because you are the one who makes the spell. Being precise is one of the things that might make a spell better because it is known that you have ti be careful with your words and precise with the sentences you say.
From the other side, there are people who find it easier to use spells that are written by authors because it has rhymes so it is easier to remember. They are more like poems. But the thing is that it is hard to find one that will match your will,desire and intent. It is also hard to let it be precise.
But basically, as i said, it comes from your own energy, is precise, match to your will and intents.
Personally I find spells that I write by my own much more useful and better.
You may choose to do the both, or just cast one of the categories.
I hope that I helped :)