Re: "actively teach" your toddler or just play and let them "learn naturally"?
Our son (26 months) learns predominantly through play and just daily life. Just describing things and asking him questions, things like that. He is always asking me, "What's that?" so he learns a lot about things that way (him being curious, and me answering his questions). Like other posters have said, it's just in how you do things. Telling them they have a red crayon or that their food is green or whatever. Asking them questions. I made games out of it, too... when DS was first learning his colors he thought it was fun when I would ask him to get specific colored blocks (he has large cardboard blocks) and he'd bring them to me and we'd stack them up and he was so excited that he knew the right colors. I really think it's best when they learn through play, and when learning is fun and not a chore.
However, my son is very interested in letters, numbers, and colors, so he does kind of seek out being taught. Like, he loves his Alphabooks (those were probably his favorite "toy" at around 18 months) and used to make me go through all 26 of them at one time. He thinks flash cards are great. He hauls out his Melissa & Doug clock so that we can count. So we do some "active teaching" but only because he shows an interest in it himself. I wouldn't push it if he wasn't so interested in these things already, but since he is then I go with it. Right now he's into his wooden puzzles, and due to the ones he has he has been learning about shapes as well as the difference between police cars, ambulances, fire trucks, etc. (and he loves when he hears a siren in real life, and he tells me "police car!" haha). He loves the number one and puts that together multiple times a day.
Just provide plenty of opportunities for your child to learn things in multiple ways, but at this age forcing it will only frustrate them. At 18 months, their job is to be a little kid. You are doing just fine!
Mommy to Grant (July 2010) and Greta (July 2012)
I purged 2,920 items in 2012! Working on 2,013 items for 2013.