The usual way to warm flash color is to add a warming gel over the flash, typically an orange/CTO gel. That shifts the flash toward a warmer tone before it reaches the film. This is generally a better approach than trying to correct everything with the lens.

A warming lens filter can also work: the 81A–81D series adds a mild warm cast, with stronger effect as you go up the series. These typically require about +1/3 stop exposure compensation.

Bouncing off a white ceiling mostly softens the light; it won’t add much warmth if the ceiling is neutral white. You may need extra exposure for bounce because of the longer light path and light loss.

One important note: electronic flash is normally a good match for daylight-balanced slide film, so if your results are unexpectedly cool, processing may be the real cause. If this happened only once, be cautious about adding warming filters immediately, or your next roll could end up too warm.