In English, there's a distinction between the word colander (for a metal bowl with relatively big drainage holes for draining vegetables) and a sieve (wire mesh). But I have the impression that Sieb tends to cover both of them equally well in German. Is that correct? e.g. say I was asking in a shop, what would be the best way to refer to them both? Thx.
It can be regionally different:
A Seiher is a colander, a Sieb is a sieve.
Colander - Seiher, Nudelsieb, or Salatsieb.
Sieve - Sieb.
Btw: Was ergibt sieben mal sieben? Feiner Sand.
Correct,at least in eastern Switzerland "Seiher" (colander) isn't used in everyday language (may be different in Germany). The distinction between Salatsieb (a colander) and Zuckersieb/Mehlsieb happens by the 1st noun of the compound, which describes the main usage.
PS:
A sieve to make mashed potatoes is called Passiersieb, the mashing is called "passieren".
A Passiersieb is normally called a Passe-Vite. Which is the trade mark name of the first such device: https://fr.wikipedia.org/wiki/Passe-vite
If asking for a colander in a Swiss shop, then you can also use the word 'LoĢcherbecken' (literally translated: plastic basin with holes) or in dialect 'es LoĢcherbecki'