You asked that the red color could be, the old Yugoslavian flag has blue, white, and red on it.
The blue represents the sky.
The white represents the air.
And the red represents Bloody Land.
So The red on the medal could possibly signify the Bloody land in which the medal could have been acquired.
Don't take my word for it, but the 1912 and 1913 medals that are the same, do not have the red, they are all gold as the attachment below will show.
So the only explanation is that your medal is reasonably rare, or that the previous owner thought it would look better if he painted it.
And for those wondering, the medal goes by the name "medalja za hrabrost, miloš obilić"