if this was an off the shelf colour and done at home rope a friend in and buy another one and just apply to the roots. Leave for the full development time.....this usually happens as the ends are lifted highly and porous. Once you have reapplied to the roots and it has lifted put an ash toner over it. Hope it helps.
Was ths result of a salon process or an at home job? If from the salon, rebook an appointment to have it corrected. If the result of a home job, consider going to a salon to correct it. In either case the yellow regrowth needs to be lightened more to achieve a paler shade prior to toning. Highlighting may also be helpful to soften the yellow regrowth, a lightner has to be used to cut thru the pigmentation.