Can I change the background color of the captcha?

Yes, you can. You can make a number of changes to the captcha settings here:

In Build 730 and higher:
/components/com_breezingforms/images/captcha/securimage_show.php

In Build 729 and lower:
/components/com_facileforms/images/captcha/securimage_show.php

//Change some settings
$img->image_width = 230;
$img->image_height = 80;
$img->perturbation = 0.9;

$img->image_bg_color = new Securimage_Color("#6495ED");
$img->text_color = new Securimage_Color("#B0E0E6");
$img->line_color = new Securimage_Color("#B0E0E6");
$img->noise_color = new Securimage_Color("#B0E0E6");