Combining data validation with conditional formatting is a powerful technique to create user-friendly and error-proof data entry forms in Excel. Here's how you can achieve this:

1. Set Up Data Validation:
- Select the cell(s) where you want to restrict data entry.
- Go to the "Data" tab and click "Data Validation" in the "Data Tools" group.
- Choose the appropriate validation rule based on your needs (e.g., "Allow" set to "Number," "List," "Date," etc.).
- Define the specific criteria for allowed values within the chosen rule.
- (Optional) In the "Input Message" tab, enter a clear and informative message that will appear when users select the cell.
- (Optional) In the "Error Alert" tab, create a custom error message that will pop up if users enter invalid data.

2. Add Conditional Formatting:
- Select the same cell(s) where you applied data validation.
- Go to the "Home" tab and click "Conditional Formatting" in the "Styles" group.
- Choose a formatting rule based on your preference (e.g., "Highlight Cells Rules" or "Icon Sets Rules").

Here's how data validation and conditional formatting work together:

- Data validation restricts what users can enter into the cell. This ensures data integrity and prevents errors from the start.
- Conditional formatting provides visual cues based on the entered data. This helps users see if their entries comply with the data validation rules. You can use various formatting options like changing cell color, adding borders, or displaying icons.

Here are some examples of how you can combine them:

- Highlight valid entries: Use green formatting for valid entries allowed by data validation (e.g., numbers within a specific range).
- Red flag invalid entries: Use red formatting for entries that violate data validation rules (e.g., text in a cell meant for numbers).
- Display icons for data types: Use specific icons (e.g., green checkmark for valid, red X for invalid) to visually indicate data validation compliance.

Benefits:

- Improved data quality by preventing invalid entries.
- User-friendly data entry with clear visual guidance.
- Reduced errors and rework in your spreadsheets.

By combining data validation and conditional formatting, you can create a more intuitive and informative data entry experience for yourself and your users.