Before jumping in to the topic of acidity and brightness, we'd need to identify the intended purpose of using those two descriptors in the coffee lexicon by first laying out what a coffee's flavor profile is.
A flavor profile describes the relative intensity of aromas, tastes, and aftertastes in a food or beverage. When it comes to coffee, there's also the consideration of the body or "feel" in the mouth dependent on consistency or thickness. In this regard, we're talking about a scent/aroma, a taste/flavor, and a feeling/texture. For example, full bodied coffee may feel "thick", "heavy", or "full" like whole milk while a light bodied coffee might have the feeling of tea or skim milk.
Acidity refers to aftertaste.
Acidity is best described as a sharpness at the front of the mouth, a radiant, pleasant aftertaste and dryness towards the back of the mouth and under the edges of the tongue. Connoisseurs say the best approach when evaluating this characteristic is to let the coffee sit on the tongue and press it gently against the roof of the mouth.
Acidity in coffee does not refer to pH levels, bitterness, or sourness.
Brightness refers to the feel of acidity.
Brightness refers to a description of a coffees acidity. Brightness is often referred to as a pleasant amount of acidity, sharp and tangy sensation leaving a dry aftertaste.
Brightness does not refer to a coffees color.
Remember, these are descriptions on the characteristics of a coffee. Acidity refers to the aftertaste a coffee leaves on the mouth while a bright acidity refers to the intensity of that aftertaste.