I primed and applied two coats of a nice blue Ben Moore eggshell latex to
some walls. Subsequently I noticed some spots that needed a little touch-up.
No problem, plenty of paint left. So I got out a brush and painted over a
few spots. I was quite surprised when that dried, to find that the
touched-up spots came out significantly paler than everywhere else.Asked
back at the friendly paint store, where I was told that "oh yes, eggshell
always does that. You can't touch up an eggshell finish -- you can only
repaint the whole wall". He seemed surprised I didn't know that. Well I had
no idea, and I've never had this kind of problem before.

So, is that correct, and if so is there no other recourse? Is it worth
trying with a roller (on the grounds that that's how the first two coats
were applied)? I don't want to make it any worse. And, if is it correct,
what's the "true" color of the paint -- the original, which I rather liked,
or the conspicuous lighter spots, which I don't like as well?

Any observations welcome,

....Robert