From the Adorama YouTue Channel:
David Bergman shows you how to get the best color out of your flash gels.
Note: Dark gels (especially red) absorb more light and will deform/melt when the gel is taped directly to the flash head and high/continuous flash pulses are used. If taping a dark gel to the flash head, it’s best to use a lower flash power and/or slower paced shooting combined with a higher ISO setting in camera to avoid damaging your gel. [Sean]