Captcha: Always spam

Hello,

This is my first site made on CodeIgniter3 : [redacted]
(it's a site where you can find the color of your car or a RAL, NCS color...)

I created a contact page, available here : [redacted]

Having a lot of spam, I have added the Captcha library, but despite this, I still have a lot of spam.

Do you know how I can reduce it?

Thanks in advance for your help


Thomas