For reference I am a happily COOL undertoned sister (purply-plum) MAC NW43-45 / Sleek Sepia. My favourite nude / natural / neutral lip colours and combos are:
Nude / Natural
Rimmel 100 Kisses lipliner in Cappucino (perfect for me browny grey mauve) + Revlon Coffee Gleam lipgloss
L'Oreal HiP Darling lipgloss (soft milky chocolate bown)
L'Oreal HiP Honesty lipgloss (sheer chocolate almost clear on my lips)
MAC Midimauve lipstick (soft apricot tinged pink nude)
Sleek Sheer Cover Lipstick Cashmere - sheer subtle golden toffee brown
Neutral
Rimmel lasting finish Zulu (d/c) cream lipstick + Rimmel Velocity lipgloss (juicy brown tinged plum)
Rimmel Provocative (d/c) lipstick
Revlon Superlustrous Lipstick Smokey Rose (Pearl 245)
Rimmel lasting finish Radiance (d/c) pearl lipstick - Very unique greyed purple plum iridescent sheer lipstick that is great in summer or when rocking that sci-fi / metallic ore look. Very wearable except can make me look a little cold if I don't warm it a bit with Rimmel Velocity or a deep plum toned liner like UD Perversion. Absolutely adore this lipstick... thought I don't wear so often these days.
Rimmel rich moisture lipstick Heather Shimmer + Barry M 6 lipliner (Coffee Shimmer is divine on warmer toned gals... more golden)
Revlon Superlustrous Lipgloss Rocker Chic (d/c) - very similar to the d/c Rimmel Velocity but sheerer and a tiny tad lighter.
*For reference / example I am wearing Rimmel Lasting Lipstick Zulu topped with L'Oreal HiP Liploss Darling. HTH