In order to change the color of the CAPTCHA you will need to add update the src for the image that appears on your form. In split view click on the captcha image. In the src attribute you will need to add in the bgcolor for the background and the textcolor for the text. You need to set these to hex color values. Here is an example of the this using the opposite of the default colors. The bgcolor is set to black (000000) and the textcolor is set to white (FFFFFF).
<img src="WA_ValidationToolkit/WAVT_CaptchaSecurityImages.php?field=Registration_group_Security_code&font=Fonts/MOM_T___.TTF&bgcolor=000000&textcolor=FFFFFF" alt="Security Code" class="Captcha" />