The media should be autoclaved and prepared before use. This reagent should not be added to molten agar media to pour plates. It should only be added as an overlay coating to solid agar media plates as described in the protocol. For colony detection: Spread 40 µL of the Blue-White Select Screening Reagent on the desired bacterial plate. Wait for 5–10 minutes to ensure the absorption of the solution. Plate the transformed bacteria to be tested and incubate at the desired temperature until colonies are formed.
Please read through this protocol on the Blue-White Select™ Screening Reagent :https://www.sigmaaldrich.com/technical-documents/protocol/genomics/cloning-and-expression/blue-white-select-screening-reagent
