Camera Labs

I will be doing a fashion shoot in early April. It will be outdoors and as I will have to travel to another country to do it, the date will be fixed to 2 days regardless of weather (does not have to be sunny, just need to avoid rain...). Which means there will be uncontrollable variables, i.e. lighting conditions as cannot wait for perfect day(just a 2 day window).

I plan to buy some shoot-though umbrellas for my:

Canon speedlite 580 plus 430 flashes, which will be triggered by my canon speedlite transmitter.

The thing I am worried about is colour temperature for the fill flash and flash in general. I have not extensively used flash, never with a gel, so want to prepare before I go.

Now the weather will either be complete cloud cover, partial cloud cover with some sunshine or sunny. The shoot will not go ahead if there is rain etc.

For the fill flash, I can probably get away with not worrying about the colour temperature. However, I like to do things right, so would like to have some advice on the best ways to change the flash temperature with either gels or perhaps other ways. Plus if it is too cloudy, I may try and create a sunny look.....

I need to prepare for all three of the above lighting conditions.

Some recommendations for light boxes and umbrellas etc. would also be appreciated.