With 50% in Class 12 CBSE boards and 81 percentile in JEE Mains, your options for college admission may vary depending on your preferences, category (if applicable), and other factors. Here are some potential college options you can consider:

1. National Institutes of Technology (NITs) : NITs are premier engineering institutions in India. With your JEE Mains percentile, you may have a chance to get admission to some NITs for branches like Civil Engineering, Mechanical Engineering, Electrical Engineering, etc. Your Class 12 percentage may not be a significant factor for admission to NITs, as they primarily consider JEE Mains scores.

2. Indian Institutes of Information Technology (IIITs) : IIITs also offer excellent engineering programs. With your JEE Mains percentile, you may have options to consider IIITs for branches like Computer Science and Engineering, Electronics and Communication Engineering, etc. IIITs often have different admission criteria compared to NITs.

3. Government Engineering Colleges : Various state-level government engineering colleges may offer admission based on JEE Mains scores. These colleges may have different admission criteria and cutoffs, so it's essential to research and apply to colleges where you meet the eligibility criteria.

4. State Government Universities : Some state government universities offer B.Tech programs and may consider JEE Mains scores for admission. These universities may have different branches and admission processes, so you can explore options in your state of domicile.

5. Private Engineering Colleges : Several private engineering colleges across India offer B.Tech programs and may consider JEE Mains scores for admission. These colleges often have different fee structures, infrastructure, and placement opportunities, so you can explore options based on your preferences.

6. Consider Diploma Courses : If you are interested in pursuing engineering but unable to secure admission to B.Tech programs, you can consider diploma courses in engineering from polytechnic institutes. After completing a diploma, you can opt for lateral entry into the second year of B.Tech programs in some colleges.

Additionally, utilize the JEE Main College Predictor to assess your admission possibilities based on your percentile and preferences.