O/L thread -
It's about keeping the bulk down (& cost) and depending on what you're going to to sew -
for thread - a spun polyester or corePolyester overlocking thread - T120s
For woolly nylon / polybulk (for loopers only) - mostly polybulk 80s available to the general market (although you can track down thinner polybulk sometimes).
There's no need to go bonkers and buy 4 cones of every colour so be selective and savvy with the combination of your choices.
The only thread you really need to colour match to your project is the thread in the left most needle. You can use your sew-all thread in the needles if you wish (sew-all thread is usually thicker than overlocking thread weights)
https://www.novatrimmings.co.uk/product-category/sewing-thread/page/2/?flpaging=1https://www.empressmills.co.uk/shop/sewing-threads/overlocking-thread/poly-overlocking/One way to start out is to get cops rather than cones, then see which colours are most useful for the colour palette you sew and then should you need to, buy cones in the most frequently used shades.
If you sew a lot of stretch fabrics, it's worth getting a few cones of woolly nylon/polybulk in basic blending shades, but before buying, get the polyester thread first and see which shades you find most useful
Overlocking thread in basic colours for starting out.
- Charcoal - which does for black as well as grey and other dark fabric colours.
- mid-light grey - which does for many greys and mid to light blues
- a light fawn/taupe type colour which does as a neutral for so many fabrics (this is a valuable blending tone for many shades and multi-coloured fabrics)
- a soft white / ivory - which does for many whites, most very light colours and light to mid yellows
Then consider what colour palette you sew and choose 3 colours in the light-mid tone range that the basics don't blend with too well. (light thread doesn't show through on dark fabric, but dark thread can show as a shadow on light fabric)
red/pink Orange/Coral/Peach Blue Turquoise/Teal Purple/Mauve Green/Sage/Lime Gold/Ginger/Brown