When loading material into your new print core, it is possible that the first string of material that extrudes from the nozzle does not match the material type or color that you have loaded.
Don't worry, this is normal. Ultimaker checks and tests every individual print core at the factory. Only when all quality checks are positive and the test print is satisfactory will the print core be packed and shipped.
These checks apply to all types and sizes of Ultimaker print cores, including those that are shipped with the printer.
If you are loading material into your new print core for the first time, simply let the printer extrude for a while. Wait until all material residue has been flushed out.
Tip: On the Ultimaker S5 Pro Bundle, the prime procedure will ensure that all material residue is flushed out automatically.