Use CyberChef (https://cyberchef.org/) to view the channels of a BMP file. Upon discovery that something seems amiss in the red and green channels, use a small python program to extract just those two channels. Then with the resulting file, use binwalk to extract the compressed text containing the flag.
g=open("output.zip","wb")
with open("output.bmp","rb") as f:
hdr=f.read(0x8a)
skip=f.read(2)
while skip:
keep=f.read(2)
g.write(keep)
skip=f.read(2)
g.close()
