Color is the primary reason, but you should also be aware that different varieties bloom as early as June and as late as September. Size of bloom will also vary from 1 inch across to 5 inches. Keep in mind that the small-flowered varieties often make up for size by offering many more flowers at a time. The ideal situation for a clematis is where it will receive at least 6 hours of sun. Look carefully at the clematis you are planting so you can see where the pairs of buds emerge from the stem.
Every source seems to have a different recommendation regarding lime and clematis.
First of all, if you are successfully growing clematis, don't worry about it. If it ain't broke, don't fix it. If you are just planting, don't apply lime unless you know your soil is acid. Most clematis grow well with a pH of 6.0 to 7.5. Much more important than lime is lots of humus in the soil and adequate moisture.