If baby is gaining weight appropriately, your milk supply is probably fine. Some experts think colicky babies can sometimes confuse stomach cramps with hunger, making them want to eat more often. Some docs recommend feeding a colicky baby smaller, more frequent meals, since an excess of milk can cause intestinal gas. Reflux can cause discomfort for babies too, making them want to eat more frequently (milk feels soothing as it goes down).
Some babies just seem to be “glued” to the boob at the end of the day. It's OKAY to let her eat several times in a row if she wants — it doesn't necessarily mean your milk supply is low. Your milk volume does tend to be lower at the end of the day (everyone’s is), but it is also more concentrated in fat and calories during this time. Evenings can be stressful (especially with a colicky baby), which can inhibit letdown too. Since baby's growth is on track, you probably don't need to take measures to increase your milk supply. However, you can always try feeding baby a bottle of expressed milk in the evenings, to see if a higher volume of milk eases the fussing. (Even better, have your partner feed baby while you pump.)