Be sure to change it back when you are done. Full-power flash will blast the subject and will look unnatural. Try a light yellow or orange gel on the flash in order to get natural-like color in the flash. A warming filter gives a nice warm tone to the subject. The gel works especially well at sunset in order to mimic the sunlight.
Cloudy days are great for flower close-ups using a little fill from a reflector or flash.
If it is extremely cloudy you may have to wait for more light. During mid-day if you have to shoot, try using a polarizer on the lens. Use the filter only at a 90 degree angle from the sun. Meter a gray card and open up from that reading. Also use the polarizer at sunset for some great effects on landscapes.