All of Herfordshire is sat on Chalk, the vast majority of it flint bearing, in fact I cannot think of anywhere in Herts that is devoid of any flint deposits. The quality of flint is not only colour dependant ie black or brown flint although black flint is considered better quality overall, thought should also be given to where the flint was collected. Surface flint can be subject to freeze/thaw action which can put lines of weakness in the material.
If you want flint for Knapping, dig for it, if you want it for sparking any surface flint no matter what the colour will do.