Do ND filters affect image quality...?

... I guess there is another layer of glass/plastic for the light to pass through, but I used a Cokin ND4 and ND8 full grad for the first time recently and noticed that the image quality was much reduced and that there was a reddish/warmer colour cast as a result. Is this normal?

Any additional filters have the potential to degrade the image but using good quality filters will minimise this.

Unusual for the colour cast to be reddish - normal Cokin filters are ND grey as opposed to actually neutral. Lee or Singh Ray are better in that they are truely neutral

Cokin filters have a reputation for having colour cast; take a look at some of the higher-priced (and better quality) filters like the Lee or Singh-Ray rectangulars or B+W, Heliopan or Hoya screw-ins (assuming you're referring to straight ND filters, not graduated NDs).