Using stand mixer fitted with whisk attachment, whip butter on medium-high speed until smooth, about 20 seconds. Add sugar and salt; reduce speed to medium-low and whip until most of sugar is moistened, about 45 seconds. Scrape down bowl, add limeade concentrate and vanilla, and mix on medium-low speed until incorporated, about 45 seconds. Increase speed to medium-high; whip until light and fluffy, about 4 minutes, scraping down bowl as needed. Add green and yellow food coloring and mix until combined.