Most commonly used colour of the filter used in dark
room safelight is:
X-ray  films  are  very  sensitive  to  the  blue-green  region  of  the spectrum and are less sensitive to red wavelengths. The red GBX-2 filter is recommended as a safelight in darkrooms where either intraoral or extraoral films are handled because this filter transmits light only at the red end of the spectrum.
Film handling under a safelight should be limited to about 5 minutes because film emulsion shows some sensitivity to light from a safelight with prolonged exposure. The older ML-2 filters (yellow light) are not appropriate for fast intraoral dental film or extraoral panoramic or cephalometric film.
 
Reference: White and Pharoah's Oral Radiology Principles and Interpretation 7th edition page no 71
Blue
Red
Green
Yellow