I've come up with a rather simple and easy way to make custom flash gels to color match a strobe to practically any light source.
1. Have a printer profile made for your inkjet printer and a decent inkjet-printable transparency stock, like 3M CG3480.
2. Shoot a white reference like a Color Checker or WhiBal card lit only by the strobe you want to match. I recommend X-sync shutter speed and f/22 and adjust strobe power to get a decent exposure. A 0 exposure setting in Camera RAW should have a well-distributed histogram that is not clipped at either end. Use a low ISO, like 100-200 to minimize sensor noise.
3. Shoot the same white reference lit only by the ambient light you want to match, with the same overall exposure level as the strobe-only shot.
4. Open both the strobe shot and the ambient shot in Camera RAW. Select the strobe shot in the filmstrip on the left-hand side, and do a click white balance on the the second-lightest neutral patch on the Color checker or an appropriate part of your white balance target.
5. Click the Select All, button, then Synchronize. The only setting you want to sync is white balance. The ambient shot will now look really ugly, but that's all good. Open the ambient shot in Photoshop.
6. Convert the file to LAB mode, and set up soft proof to proof to the transparency printer profile with Absolute Colorimetric rendering intent. Use the eyedropper to select the color of the white reference. Configure the Info window to display the original image values and the soft proof RGB values. Use the Pen tool to create a shape, and then delete the vector mask so that it fills the entire document. Using the info window as a guide, adjust the L value of the shape color so that the highest soft proof RGB value is in the high 240's. This will ensure that the gel you're about to create is saturated enough to do the proper color correction, but does not decrease your strobe's output any more than absolutely necessary.
7. Adjust the canvas size as necessary to get the gel size you want, then print your color layer to the transparency film, using the appropriate printer profile. You now have a custom flash gel to match your light source. Here's a quick sample of mixed incandescent and flash gelled with the procedure above:
Note the absence of any color cast where the shadow of the laptop display falls on the whote foamcore on the table, as well as the CRT monitor shadow on the wall. Ordinarily there would be an orange/yellow cast to the shadows if the WB was set to match the flash, or else the non-shadowed areas would have a strong bluish cast if WB was set to match the incandescent lighting. With this technique, the color temp of the flash matches the incandescent ambient light, and that means one white balance setting works equally well for both flash and ambient. If you shoot events in venues with weird light (like a mix of incandescent and fluorescent with strongly-colored walls), this could really improve the color balance of your flash photos.