The challenge of this painting project is use a limited palette of two complementary colours, plus white, to paint the tubes you’re using (or the pans of watercolour paint). Also to paint the tubes larger than actual size and from life, not from a photo of them. Painting the lettering on the tube is optional.
Complementary colours when mixed together produce greys and browns. Mixed together without white, they’ll give you the darkest version of themselves, which can serve as your “black”. With the addition of white, you’ll really start to expand the range of greys and brown.
My favourite complementary combination is blue and orange. The other pairs of complementary colours are yellow and purple or red and green. Exactly how they mix depends on which blue etc you’re using, as every pigment mixes differently. That’s part of the joy of using a limited palette, that you get really familiar with a colour’s personality and build up instinctive knowledge of how it’ll mix. (To find out what pigments are in a tube colour, have a look at the tube label or the manufacturer’s colour chart on their website.)

Tip: Unless you have spare tubes of the two colours you’re using, mark the surface the tubes are sitting on so that you can return them to the same position if you need to squeeze some more paint out. A pencil mark around or some tape, will do the job. Unless you’re working on a ginormous scale, squeezing extra paint out of the tube won’t change it’s shape dramatically. And if it does, simply adjust your painting, or not, as you wish.
Suggestion: If working with mixing complementaries is new to you, it’s worth spending a little time mixing up the colours in various proportions, and with an increasing amount of white, to see the range you can produce. This will allow you to focus only on the mixing, rather than tackling new colour mixing and capturing a subject simultaneously.
Remember: If you’d like personal help with creating your project painting and/or personal feedback on your finished painting, this is available to all my project subscribers via my Patreon site (click here). To help my creating the projects, there’s also a supporter’s tier on Patreon.
Have fun, and do send me a photo of your painting for inclusion at the bottom of the project page for us all to enjoy, or share it in the Community Section of my Patreon site.
For my painting, I used Schmincke Prussian Blue, which is my favourite blue (it’s a mixture of two blues and a black pigment, PB60 / PB15:1 / PBk7) , and Schmincke Transparent Orange (PO71) which is darker than cadmium orange(and, as the name suggests, transparent rather than opaque). Plus Golden titanium white (PW6). Every colour in my painting below comes from these three.

To watch over my shoulder as I paint this, watch the video below (link to this on my Vimeo page). There isn’t any sound or explanation on this video, simply me and three paint colours.
If you’re thinking “paint a tube” seems a familiar project, you’re remembering this project from 2019.