white ABS masterbatch usually has bluish or greenish color. optical agents lead to green tone in white masterbatch and omit inherent yellow color of ABS but will decompose during the process.
I assume you mean emit rather than omit the un-desirable yellow. In this case using the terms ‘additive’ and ‘added toning’ are misleading because the process in question is ‘subtractive colour mixture’. Your white ABS reflects all spectral wavelengths approximately equally but the yellowed sample absorbs some of the blue light. Optical brightening agents (OBAs) use fluorescence to add back the absorbed blue light and thereby correct yellowness and the fluorescence itself may be greenish. The florescence can however easily be quenched by contaminating impurities and quenching and evident greenness may also occur if excessive amounts of OBA are added.