Colour-sensor(I2C slave) <--> FPGA (I2C master + main_design)
Please understand that a full-scope project development support is not the objective of these forms and is also not possible. You ask specific questions and you will get helpful replies.
Break down your project into multiple parts:
1. I2C master - there are freely available cores, get one and implement it.
2. I2C slave (your colour sensor) - Verify if it can communicate with <1>.
3. Check if the main_design can read the data register of the I2C master (which should be populated by the data sent by the colour sensor).
4. Now it is your game what to do with the data (e.g. - glow various on board LEDs as per the register data, etc)