There will never be a box of artist's tools--crayons, pastels, pencils or paints--that comes with every colour under the sun. You cannot paint a nature scene with just a few unmixed colours and get the image right. Even painting one object may require the use of different colours to create the effects of light. Every artist has to mix colours to get the shades she wants. One precise colour is claret red. Claret red is the colour of a particular type of red wine and is intensely dark red with a hint of purple.
Mix additional colours into the first colour, which is the base colour. If using paint as a medium, just dabbing the very tip of your brush bristles into an additional paint can be all the colour you need to add to get the desired shade.
Take a base true red and add some umber (brownish red) and a touch of black.
Combine two parts carmine (or alizarin red, a dark pure red) with one part ultramarine blue (a deep, rich blue).
Add a touch of vermillion (bright red) to enhance liveliness and prevent muddiness. A bit of yellow can drab the colour a tad if it gets too bright.
You can always add more paint to achieve your colour change, so add additional colours a bit at a time and check the effect between additions of colour. It is often better to add another colour to darken a paint than it is to add straight black, or to tone the black with a colour first. Consider the undertone of the shade (secondary colour) you are trying to achieve and add a darker version of the colour you need. For example, to get dark green you may wish to add dark blue to yellow instead of black to green.