Cotton fibre naturally contains wax, pectins and impurities, and knitted fabric carries oils from knitting. If they are not removed first, the dye penetrates unevenly and patches, streaks and dull shades appear. For this reason fabric goes through preparation: alkaline scouring (cleaning), then bleaching for pale and pastel shades.
Fabric prepared for dyeing is called PFD (prepared for dyeing) and has even absorbency. For dark shades the preparation can be lighter, but it is never skipped. Poorly prepared fabric gives large shade differences that cannot be corrected after dyeing. Ask whether preparation is part of the standard process.
pfdpreparationscouring
