What exactly did they complain about? You've complete control over how the captcha displays via its parameters in CB AntiSpam > Parameters > Captcha > Modes > Internal > General. The new defaults are as follows.
Image Height: 50
Image Color Range: 0
Image Background Color: 255,255,255
Image Text Color: 20,40,100
Image Background Noise: Enable
Image Background Noise Color: 20,40,100
Image Foreground Noise: Enable
Image Foreground Noise Color: 20,40,100
Image Font Type: EARWIGFA.ttf and monofont.ttf
Character Rotation Range: 13
Character Offset Range: 3
Character Size Range: 5