Combination pills are most often monophasic, meaning that all 21 pills contain the same amount of estrogen and progestin. To make this clear they all have the same colour. But there are also pills that are bi- or triphasic, which means that the dose and kind of hormones vary during the three week period. Normally the pills in each phase have a different colour.
One kind of biphasic pill is the ‘step-up pill’, with the first 11 tablets containing less progestin than the rest of the pack. Another kind is the ‘sequence pill’: the first 7 tablets contain only progestin, while the next fifteen contain both estrogen and progestin. In each phase the pills have a different colour.
 The triphasic pills consist of three phases: each having a different colour. The amount of progestin increases.