What I generally recommend for all clients I’m about to capture is to wear neutrals (greens, browns, cream, khaki, etc.) or soft pastel colors. It’s not entirely necessary but it definitely compliments colors in nature, bringing the focus primarily on the subject.
Clothing should not be form fitting and shouldn’t be too “busy.” Subtle floral patterns can sometimes be nice but it’s best to avoid other patterns like polka dots, pinstripes, and color blocks. Men should avoid t-shirts and choose collared shirts (Henley shirts work if they're the stubborn type).
Say NO to smart watches, colorful and bold accessories, vivid attire, pocketed cargo pants/shorts, “trendy” shoes, and fashion logos.