The ground colour is the background colour as against the colour of any markings on it.
For examples.
The ground colour is blue-black with parallel stripes of ashy blue on both wings.
https://live.staticflickr.com/4670/3...4d2f7789_b.jpg
The ground colour is heavily dark-shaded.
http://3.bp.blogspot.com/-f3wB2A2ujB...0/_MG_1531.jpg
The orange HW band contrast strongly with the ground colour.
http://www.butterflycircle.com/check..._female_01.jpg
The HW band barely contrast with the ground colour.
https://1.bp.blogspot.com/-jNl8Waf_H...ale_03+(1).jpg
TL Seow: Cheers.