Culculate: This term is used when the petals are in a more upright cupped form but not as open.
Cupped: Cupped is referred to when the petals form a cup as in a tea cup.
Flat: This refers to petals of each flower being flat when mature.
Reflexed: Reflexed is when the petals pass below the horizontal line of the flower.

Single: each flower has 4 distinct petals set on a single layer
Double: each flower has 2-3 sets of layers of petals.
Multi-petal: these flowers are usually single but rather than have 4 petals in a row, the flowers may have from 5 to as many as 40 petals arranged on a single row.

Lilacs are broken up into 7 distinct color classifications. The color is taken at 1/3 bud break stage of the petals and a guide is used to determine the color. Colors will change in lilacs from minute to minute as the sun rises and again from day to day as the mature out. 

Also important to understand that there are no clear cut colors when we speak of lilacs. 

A red color can have many tones and hues of magenta. Each person will have a different interpretation of colors also. 
Below will are some examples of each category.

Again flower colors are complex and they can have many streaks, tones and hues mixed in of other color categories.