1. Look at the original code - you need this part -
Code:
onmouseover="javascript: dicon_over.call(this, '{$i.thbn_url|amp|escape:javascript}', {$i.thbn_image_x|default:$product.image_x}, {$i.thbn_image_y|default:$product.image_y}, {$i.is_png});"
3. change the colorbox setting
opacity0.85The overlay opacity level. Range: 0 to 1.