Henry David Thoreau once said, "...a man must see before he can say." Our nature, as human beings, is to zero in on visual clues that are most obvious while overlooking more subtle information. In the case of flower identification, color is uppermost in our minds as the cue that jogs our memories, so we naturally fall into the mistaken belief that it is the most important of the characteristics we use to identify plants. In fact, it is probably least significant of all a plant's features, so go ahead and enjoy those lovely colors, but look elsewhere for clues to its name.
Carry a notebook and pencil with you and use it to jot down important observations about the plant in question.
Count the number of petals. Note petal arrangement--in a circle or "wheel" pattern they are referred to as radially symmetrical, or regular. If not symmetrical from one half to the other when divided down the center in any direction, they are referred to as bilaterally symmetrical or irregular.
Notice how flowers are arranged on stems--one flower per stem, multiple flowers on each stem, flowers clustered together like an umbrella, flowers in a long cluster or spike--these flower arrangements are critical to assessing a plant family for further identification.
Measure the flower. It is not necessary to use an actual ruler, but try to get a general approximation of the overall width and length. Determine height and spread of the entire plant as well.
Examine the leaves. Are they round, heart-shaped, oval, strap-like, lobed, etc.? Do they alternate on the stem, spiral around it or only cluster at the base of the plant? Answer those questions to narrow the identification of your plant further. Check leaf edges, veins, hairiness and so forth for more hints.
Note the season of bloom. Plants bloom in specific, often narrowly defined seasons, so look only at plants in your guide that bloom during the season in which you discovered it.
Look at the environment around you--notice the general conditions of sunlight or shade, wet or dry soil or other conditions that might affect plant growth. Plants need specific conditions to thrive, and these are important clues to identification. Determine the basic biome or habitat to narrow choices (i.e. prairie, woodland, swamp, etc.).
Use a field guide to flowers for the area in which you are located, in conjunction with all the information gained in this examination, to determine which purple flower you are looking at.