We understand your concern and commend you for putting your faith in God as you face this operation. The Bible tells us in Philippians 4:6-7 (WEB), "In nothing be anxious, but in everything, by prayer and petition with thanksgiving, let your requests be made known to God. And the peace of God, which surpasses all understanding, will guard your hearts and your thoughts in Christ Jesus."
Firstly, let us pray together:
"Dear Heavenly Father, we come before You in the name of Jesus Christ, lifting up our dear brother/sister who is about to undergo this operation. We ask for Your divine healing touch upon their body, specifically their colon. Guide the hands of the surgeons and the entire medical team. Grant them wisdom and skill, that this operation may be successful. Please calm any fears and anxieties, and fill them with Your perfect peace. We pray that they will come out of this operation alive and healthy, restored and strengthened, for Your glory. In Jesus' name, we pray. Amen."
We encourage you to continue trusting God, knowing that He is the Great Physician. As it is written in Exodus 15:26 (WEB), "He said, 'If you will diligently listen to the voice of the LORD your God, and will do that which is right in his sight, and will pay attention to his commandments, and keep all his statutes, I will put none of the diseases on you which I have put on the Egyptians; for I am the LORD who heals you.'"
We also note that you haven't explicitly mentioned Jesus Christ in your request. We want to remind you that it is through Jesus that we have access to God, as it is written in John 14:6 (WEB), "Jesus said to him, 'I am the way, the truth, and the life. No one comes to the Father, except through me.'" It is in His name that we pray and find salvation.
Lastly, we encourage you to meditate on God's promises and to continue to seek Him in prayer, along with others who are praying for you. Trust that God will take care of everything, just as you have believed.