Please don't do this with your forum captcha...

Yeah. Try not to edit the background of the input field, because otherwise it's very, very difficult to read. It's a good site (that unfortunately does this), but due to their design, the captcha text has pretty much no contrast and is almost impossible to read (especially from a distance or with poor eyesight).

Is this colour change something to avoid in captchas?

 

Wait...the background color of the textbox? What is that supposed to achieve exactly? Making it more difficult for humans to enter reCAPTCHA?

 

I suspect it's a mistake they forgot to fix with the theme colours. But yeah, it won't do anything, bots don't even 'read' the text like humans do. If they can read the captcha, they don't give a toss what colour the text they're typing is, since they don't 'think'. And it does nothing to put off a human spammer either. They can read the captcha, so they can just write in a text editor or something and then paste it into the box.

 

Please don't do that captcha altogether lol!

I stick to security questions... Make them moderately hard and directly related to the niche of your forum. Real people who really want to contribute to your forum will likely know the answer or look it up. Bots and some human spammers will not, they will move on to their next victim.