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Many warm colors have a cool undertone and you are probably using a cool white as well. Titanium White, now most commonly used in place of Lead White, is cool, whereas Zinc White which is often shunned for its bad reputation as an oil color is warmer. Some people, however, use Zinc as their mixing color as its lesser opacity to Titanium White is desirable when delicacy is called for. In anycase, the natural cooling effect of any white can be offset by a small addition of the next lightest warm color in the range. Some people also follow the practice of darkening colors with the next color down on their own scale to offset the de-saturation caused by adding a relatively neutral black.