Strengthen it with practice. Find a friend willing to practice with you and have them sit across from you and clear their mind. Then ask them to think of a color as hard as they can, but not tell you what color it is. They should think of the color in all ways, mentally saying the name of the color over and over while they picture a field of the color in its best pure form. It's easiest at first to limit this exercise to the basic primary colors (red, yellow and blue), then when you are feeling more confident have them start using both primaries and secondaries (red, orange, yellow, green, blue, violet). Finally, once you are feeling confident with the second group of colors, ask them to start adding more specific colors ( brown, pink, tan, dark green).
As you are trying to sense the color they're thinking of, it will feel like you're pushing your own mind into theirs. You might see a flash of the color they're thinking of, or hear their mental voice saying the color's name. Don't try too hard with each one, it's too easy to second guess yourself and you can always try again if you get it wrong.
The reason you should start with colors is so you can get very pure thoughts to sense. Most people have a lot of background noise in their minds at all times (I'm sure you know what I mean..."what do I want for dinner"... "I really like so and so"... "I'm worried about my physics test.") Having them focus on a color minimizes that noise because colors are simple concepts.
After you master colors, you can move on to some more complex subject lke animals. I don't suggest it till you're good at the color exercise.
Oh, and don't get discouraged if you can't do it at all at first. Unconscious telepathy (like what you described) is 1000 times easier than conscious controlled telepathy. You can do this, but don't ever expect 100% success rate either. I consider myself pretty good at it but I only get it about 80% or so. It's hard, hence the reason most people don't bother.
Good luck and feel free to mail me questions if you have them.