Using a numerical value, the easiest way to make your skin colour invisible without a program is to add 16777216 to the current value.
Pick the colour you want (using paint or whatever).
Write down the red, green, and blue values.
Then calculate (red * 65536) + (green * 256) + blue.
Add 16777216 to the total.
That should give you an invisible skin, that changes to the colour you first picked when you're damaged.
I haven't tested this fully, but it seems to work for me. Try it and let me know.
I always use numerical values, but it may be possible to use another method.