It is generally safe to paint indoors when it is rainy and 66 degrees outside, as weather conditions outside should not significantly impact painting inside. The optimal temperature for indoor painting is typically between 60-80 degrees Fahrenheit, so 66 degrees falls within this range. Rainy weather is not a concern when painting indoors, as the controlled indoor environment provides stable conditions for paint to dry properly.

However, it is important to ensure proper ventilation when painting indoors, regardless of the weather outside. Opening windows or using fans can help facilitate the drying process and disperse any fumes from the paint. Additionally, make sure the humidity levels in the room are not too high, as excessive moisture can affect the drying time and overall finish of the paint.

In conclusion, painting indoors when it is rainy and 66 degrees outside is generally safe as long as proper ventilation and room conditions are maintained. It is always recommended to follow the specific instructions provided by the paint manufacturer for optimal results.