could the US change its currency to a different color, shape or size?

The U.S. Treasury is currently introducing new currency with differnet colors, inks and additional security features into the market. There are also gold colored dollar coins, in addition to silver. So far, it has not affected the role money plays.

Look up the US Treasury site for more information about the surprisingly interesting history of currency designs and plans for the future.

No, it would not affect it at all.

The paper itself, has no value. Unlike coins, that hold it's own value.