Colours are intentionally inconsistent. Dorico uses one set of colours per instrument, but separate voice names and numbers per stave. This means that Upstem Voice 1 on the top stave of a piano will always be different to Upstem Voice 1 on the bottom stave of the same piano. This rethink means that cross-staffing is an absolute joy, where in Sibelius it’s a total mess. Note that if the two Upstem Voice 1s on the two piano staves were the same colour, you’d be unable to see which stave the voice originated in, and this would lead to a whole different set of problems. Regardless, Dorico doesn’t care if you’re using Upstem Voice 4 or Downstem Voice 6, if that voice is the only one present on the stave, so you needn’t care either.
Numbers are easy as long as you know that a) if there’s only one voice the stem directions will follow the usual rules and b) Upstems have their stems pointing up (from the right side of the notehead), downstems have stems pointing down (from the left side of the notehead). When entering notes, use V to cycle round existing voices on that stave, and Shift+V to create new voices. As long as you don’t misuse Shift+V you’re unlikely to get into a mess.
As to “dealing with Starts Voice End Voice”, Edit > Remove Rests is almost always quicker.