One word: contrast
Everyone else has given great examples of this, so no need to repeat it. But it's a concept I failed to realize for a long time. Making subtle shades and highlights is great and all, and seems intuitive at first. But really contrast is how you make a model pop. Ben Komets, an awesome painter, said that for red color specifically it's best to start with the brightest color you want then shade downwards, as opposed to starting dark and shading up. Reason being is that when most people shade up with red, they often use oranges and yellows to highlight -- which is fine -- but will make your model more "apricot" or pink. I followed his advice and it works amazingly.
Watch this and it will change your life:
https://www.youtube.com/watch?v=E7dmPDrWfuI