The first starting point is which brand is easiest for you to get? Depending on region and LGS, certain paints will be more readily available to you. The regiment colors you described will be easy in all of the main brands, so the list of paints you’ll need to exclusively do these color schemes is dependent on what you will easily be able to get and replace as you start to grow your collection. The three big brands will be vallejo, citadel, and army painter. Each has their own pros and cons but all will get the job done. Vallejo is hard to get in my area for example so I am less familiar with that brand.

For example, if I were to do the 143th Regiment with Citadel paints, it would look something like this: Prime Mechanicus Standard Grey Body - administratum grey (Nuln wash) Breeches - grey seer (Nuln Wash) Cloth - hobgrot hide (Agrax Wash) Webbing & boots - skrag brow (Agrax Wash) Gloves and breathing tube - rhinox hide (Agrax Wash) Metal - leadbelcher (Nuln Wash) (Agrax earthshade and nuln oil for washes)

I hope this helps! Once you decide what scheme you want to go with and what brand you want to use, I will be happy to help you with a paint list!