Problem: Find the flag being held on this server to get ahead of the competition http://mercury.picoctf.net:45028/
Hint 1: Maybe you have more than 2 choices
Hint 2: Check out tools like Burpsuite to modify your requests and look at the responses
Solution:
Using Burpsuite you learn that clicking the red button sends the request as GET and clicking the blue one sends the request as POST.
The title is the hint. You need to send the request as HEAD.
Then you have the flag.
Flag: picoCTF{r3j3ct_th3_du4l1ty_775f2530}