Solution Code

#!/usr/bin/env python3
header = '''<?xml version="1.0" encoding="UTF-8" ?>
<svg width="1920" height="1080" viewBox="0 0 1920 1080" xmlns="http://www.w3.org/2000/svg" version="1.1">'''
footer = '</svg>'

c1_c3_c6 = []

with open("flag.svg", "r") as f:
    content = f.read().split('\n')

    for i in content:
        if "c1" in i:
            c1_c3_c6.append(i)
        if "c3" in i:
            c1_c3_c6.append(i)
        if "c6" in i:
            c1_c3_c6.append(i)


with open("sanitized_flag.svg", "w") as f:

    f.write(f"{header}\n")

    for i in c1_c3_c6:
        f.write(f"{i}\n")

    f.write(f"{footer}")

Flag: sdctf{c0untle55_col0rfu1_c0lors_cov3ring_3veryth1ng}