T.D.
I was in your exact same position 3.5 years ago. My daughter was three and my son was a newborn. I don't know if my son had colic, but every night for about a month, from about 5:30 to 10p he would cry and cry and cry. I had to walk him all around the house while he screamed in my ear.To make matters worse, my husband worked nights, so it was just me and the kids.
Mostly I just told my 3yr old ALL the same things you are saying. (Your brother is little and small babies don't know how to communicate other than crying. He is not sick and he won't always be like this... etc..) My daughter was very accommodating and sweet and gentle, even when I had to elimiate the bedtime routine she and I used to share (ie., reading books, etc.) because her brother was HOLLERING and turning purple!! On the nights my husband was off, he gave her lots of EXTRA attention - took her out to dinner or just read to her for a long time so she got some individualized attention. I also tried to give her extra attention when the baby was sleeping, even though I was excited or during his non-cranky times...just like you're doing.
My daughter had no negative impact from this time in our lives. Kids are so resilient and having a new baby is probably so new to your older son that he probably thinks this is normal! In six months he may not even remember these early weeks. I know these nights seem interminable now, but things do get better. Right now, my son is 3.5 and he is super sweet and super laid back, so this early colic is not necessarily a precursur to future behavior. Hang in there, you are doing everything right.