I'm tempted to translate a manga for the folks who don't understand it because ahhh it's so cute and needs more fans but I don't have the time for that (making money > studies > art > anything else)
Even though a shiny Venusaur is green, a Nest Ball would look like garbage with it because it has the wrong shade of green. Not to mention a Nest Ball gives off a more foresty feel... now a Quick Ball would be the best option. A Quick Ball would give it a better tropical feel and has yellow on it, both of which match the rafflesia on a shiny Venusaur's back.