if the second coin is the same color as the first then you could just do it very casually as if it doesn't matter while the main focus is elsewhere and close their hand for them quickly like in a sponge balls routine. Here's how I would do it.
-shuttle pass, "could you put your hand out sir",start to put the coin in their hand behind cover of my fingers. Now depending on the context of the trick i look at something else and mention it and they look as well as i finish placing the coin in their hand and close their fingers with my thumb then turn their now closed fist over. It doesn't take much sleight of hand at all, its all pretty much misdirection. Hopefully this was helpful or at least gave you an idea on one way to do it. edit- now that I think about it the coin is never in view after the shuttle pass so it doesn't even matter if it's the same color or not.

-connor-

Thank-you! That is EXACTLY what I wanted to know how to do! Thank-you very much! If anyone else has anything to add to this guy's post, PLEASE post it!

I was pretty much saying what acebrawler siad, he was just nice enough to give a good example of it

I still recommend the Bobo switch though. The spec will see you hold a coin in your hand and plainly drop it in the other. They actually see the coin fall and go into the hand, which is good for switching different-looking coins. They won't notice the difference in color because it's just a split-second move. I only use a shuttle pass when working with identical coins.

Ooh sorry , please forgive me kraynik .

By the way, I know what exodus said to get but if I buy J.B. Bobo's Modern Coin Magic, will it teach me how to do this?

I'm looking through it right now and haven't found anything like the effect you want, but it will definitley teach you every switch, subtlety, and theory you need to pull it off.

EDIT: Yes, I found some tricks like that. "The Ghost of a Coin" and "Silver Extraction" both involve a coin changing to a transparent disc in a spec's hand. Ironically enough, they both you the Bobo switch

yea, a bobo switch would probably work better. I was just going off of what google521 had said before. Or a one handed switch,(almost the same thing but i think it looks better). Anyway I'm glad i could help.

-connor-

Woot! I just made up a little trick, just to try putting a coin into a spectators hand and it worked. They didn't see me putting a different coin into their hand, except they thought I was holding a coin on my other than threw the coin from. But, I secretly slipped it into my pocket and then they were like,
"Huh! I thought you had a coin in that hand! Where did it go?"
and them I'm like,
"You made it disappear and change into a smaller coin ."

I know how you outt a coin in some body's hand with out them noticing it...

and it's pretty easy to.