Affection is not only permitted while you are niddah, it is encouraged. The problem is with using playful affection which is sexual or arousing. For this there aren't really clear cut rules, as each couple is different in what creates arousal. Even an innocent comment in a certain context or setting can be arousing for certain couples, and for that you need to know yourselves and each other. Certainly being not only civil, but loving and caring for each other are essential for building and maintaining the relationship, particularly during niddah. The only absolute prohibition would be actually speaking about sex or sexuality and desire in a way which would lead to arousal. Beyond that, it is a matter of getting to know each other and yourselves, and having restraint where you know it is necessary.