The colour I can't do without? My regular students will be able to answer this question for me without hesitation. They refer to it as 'Carol's purple'.
When teaching, I encourage my students to carefully select strong darks and bright lights to create a good contrast within their paintings, regardless of the subject matter. The 'dark' that I turn to most readily is purple.
Very often, the main subject of my Painting includes tans and/or gold. As you know, purple lies opposite gold on the colour wheel. Opposites on the colour wheel are known as complimentary colours and they always work well together. Purple and gold used together can bring to life an otherwise dull scene. Of course, it doesn't mean that my paintings don't have other colours, simply that these two colours dominate.
Now somehow I manage to see 'purple' in most subjects that I paint.
For your information, my favourite purples are the sennelier 'purplish-blue-grey' range.