This is totally normal. Decolour Stripper is not a colourant but a formula designed to remove both natural and artificial pigments from the hair. When an apricot or yellow shade is revealed, it is indicating all the colour pigments have removed and the colour of the hairs natural keratin fibres has been exposed. Keratin is yellow.
How to remedy
You must now proceed to the second step and re-colour the hair to the desired shade. If a pale or platinum blonde is desired, use Colour Restore Iced Platinum on this yellow hair to tone to a white blonde result. If the hair is naturally quite dark and has stripped to an apricot canvas, using a lightening blonde colourant featuring a cool tone, will also create a light blonde result.