Which chemical is utilised during the isolation of $DNA$ at the end of precipitation?

In Gene cloning:

In the presence of a chromogenic substrate,what color colonies do recombinants and non-recombinants produce,respectively?

Elution means

$A$ recombinant $DNA$ molecule was created by ligating a gene to a plasmid vector. By mistake,an exonuclease was added to the tube containing the recombinant $DNA$. How does this affect the next step in the experiment,i.e.,bacterial transformation?

Silencing of specific mRNA in RNAi is by