Cloudy and dull weather or overcast days makes for EASY lighting to work with. Your shadows will be open and diffused, our exposure metering can be off a stop either ay and the results will not be too adversely affected. Becasue the cloudy weather will make the shadows open,not deep,dark and inky-black, you CAN if you like, boost your saturation and tone curve (the digital equivalent of contrast) a bit in-camera, or do that later in post-processing.
If you do not have the software to post-process,and you are going to be shooting in JPEG mode in the field, perhaps boost your tone curve (ie, contrast) a bit, perhaps 2 clicks above normal, and also add a bit of saturation as well. And perhaps notch the sharpness up a tad bit too.
Not being familiar with your camera, perhaps you could set it to a "scene mode", such as "enhanced color" of "vivid" mode, if you have such a thing. One area where there could be problems is when shooting toward a large expanse of bald,white sky that is backlighting people or subjects that are in somewhat of a shaded location--be aware that such lighting looks awful oin digital,and you'll need fill-flash is the shot has large expanses of white sky in it, or else risk silhouette effects or ugly color.
As far as the 18-55 versus 70-300 lens: it's a toss-up. Some people feel that they get better candids shooting in plain sight and from very close range, and they disparage what they call "sniping", or shooting from longer range with a telephoto or tele-zoom lens. Some of it depends on the nature of the crowd and the culture of the area. OTOH, for stage performers and people participating in events where there are LOTS of people pointing cameras at the same subject, like say a 1-on-1 oyster speed shucking contest, the 75-300 would give you a LOT better close-up shots when shooting from the crowd. Also, a 75-100mm focal length on a DX body means a VERY narrow angle of view,and to capture a lot of scenes, the FOV factor will physically limit you to shooting distances of 25 feet, or farther, and yo simply will not be able to use the tele-zoom at normal distances. My suggestion? A fanny pack or hip holster to hold the 70-300 for when you don't need it. Take both lenses if you want to cover all bases.