Warm up filters are splited into categories: 81 & 85
81A, 81B, 81C are light balancing filters used to decrease the color temperature slightly for a warmer (redder) tone. Corrects the tendency toward bluish tones. For example, the 81A should be selected when using tungsten Type B color film (3200°K) with photoflood lamps (3400°K) These filters can be used together.
85, 85B, 85C are color conversion filters for the use of tungsten type color films in daylight. 85 decreases the color temperature from 5500°K to 3400°K for the use of Type A color films. 85B decreases the color temperature from 5500°K to 3200°K for the use of Type B color films. 85C decreases the color temperature from 5500°K to 3800°K. The effect obtained is the same as with daylight type color films used in daylight.
Some places need warming up, such as cities where there is a lot of concrete. I use an 81A all the time as portraits work better with them generally. If you are using digital then this is not a problem. I used to always shoot a white card for the first frame of a shoot to balance any colour temp factors.