DB’s theme is yellow and white. It makes sense that there would be a lot of yellow skins. As for green, it’s a colour that’s always associated with the military. Combine yellow cartoony plastic with the standard green military style, and you get the turtle and phoenix obsidians. However, this is most noticeable in containment war skins, which are literally based on textures from the CDA’s containment shield.
Personally, I hate the combination of green and yellow. The turtle obsidian was hideous to me, and I’m almost considering not buying the phoenix card just because of the gun skin.
It’s worth noting that only so many colours can actually work for skins though, yellow just happens to work very well. But it’s not the only colour recently. the phantom obsidian was black and orange, twitch skins are purple, hexagon skins are light blue, etc.