Tried it. I tried it in two ways (probably not the right ones).
1- I take my original rival sprite and convert it in 16-bit with Irfanview. I take the 16-bit sprite and paste it on the BLUE sprite. Results: Same problem as before.
2- I paste my original rival sprite on the BLUE sprite. I save and convert it in 16-bit with Irfanview. Results: Worse than before. All the colours are screwed up. None match and there is no transparency.
What did I do wrong?