Here is what I found. George Washington originally implemented the Puple Heart for Military Merit, not being wounded on the battlefiled, so it seems the award originally was just a cloth Purple Heart with Gold embroidery. It was not until many years later that it was reintroduced as a award for being wounded on the battlefield. So the Purple color has nothing to do with injury. Not sure why Purple was chosen.
Here is some info I cut from another web-site with some history of the medal.
The Badge for Military Merit
The original Purple Heart award was instituted by George Washington in 1782 to reward troops for "unusual gallantry" and "extraordinary fidelity and essential service." The award was a purple cloth heart edged in silver braid, and was to be worn over the left breast of the uniform. Only three awards are known to have been issued, of which two are known to exist today.
One of the two known examples of the Badge for Military Merit.
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The Modern Award
Pre-WW2 Awards: The Purple Heart as we know it today was reestablished in 1932 to coincide with the 200th anniversary of the birth of George Washington. The original criteria for award of the Purple Heart as published in the War Department Circular No. 6 of February 22, 1932 states that the medal be awarded to anyone serving in the Army who had received combat-related injuries or had received the AEF's Meritorious Service Citation Certificate during WWI, the latter criteria harkening back to the intent of George Washington's "Badge of Military Merit".
Although this awards was retroactive to any soldier wounded from the Civil War on, eligible recipients were required to submit a formal application to the War Department for approval before the issuance of the medal. In 1942 the Army estimated that approximately 186,000 living veterans were eligible to receive a retroactive Purple Heart. Approximately 78,000 retroactive Purple Hearts were awarded between 1932 and 1942.
While the award of the Purple Heart was not authorized by the Navy until 1942, sailors and Marines who had been wounded prior to 1932 were eligible to apply for the medal. While no numbers are available concerning the award to Navy and Marine personnel, approximately 12,000 sailors and Marines were wounded between the Civil War and the Nicaragua Campaign of 1932.