Best colour for a dog to track an object against green background
Dogs are a dichromatic species, featuring only a long wavelength (L) and a short wavelength (S) cone (source: Smithsonian). As such, they are thought to perceive mainly blues and yellowish hues (Fig. 1). This is unlike trichromatic species like humans, who are able to distinguish red and greens as well (Fig. 2). Whether dogs perceive greens or yellows is hard to say, but either way, they see those longer wavelengths (red, green, yellow) as one hue (say, yellowish).

Hence, in dogs, green hues are basically seen as yellow (or vice versa) and indeed blue (short wavelengths) are best to create a good color contrast.

Of course, brightness contrast will work fine too (white and black).


Fig.1. Dichromatic (dog) color spectrum. source: Dog Vision 


Fig. 2. Trichromatic (human) color spectrum. source: Dog Vision