Remember when using white to lighten your warm tones that it will cool, deaden and dull your warm hues.
In the illustration provided, the onion reads as white, but it has a hint of cool and warm color notes within its overall shape. If it was painted pure white, the object or highlight would dominate your painting way too much.
So, how can you stop making pink tomatoes? The key is when attempting to lighten warm colors like reds, oranges, etc., add yellow to start the lightening process. Yellow is very high on the value scale and will help elevate your color to a higher value. If you need to add more white to get the value even higher, then add just a touch of the warm base color you are working with back into the mix to compensate for the cooling action of white.
Pure white will move forward and dominate in your artwork, so only use pure white for the extreme bright highlights in your painting and use it to a minimum. It is safer to make any of your whites “almost white” by adding just a touch of color to add warmth to your highlights, mid-tones and darks.