Colours are usually kept the same across different banknote series, because people are already familiar with them, and it makes things easier when switching to a new series. In fact, many of the colour choices go back to colonial times. The British-era notes used blue for $1, green for $5, red for $10, blue for $50, grey for $100, purple for $1000, and green for $10000. When Singapore issued our own Orchid series, our government kept most of these colours, with only minor changes: the $100 became a bit more bluish than grey, and two new notes were added—$25 in brown and $500 in light green. The Bird series followed the same colours, except the $20 was changed from brown to yellow. In the Ship series, some colours were adjusted. The $2 note was introduced in orange but was later changed to purple because people confused it with the red $10 note. The $100 took on a brown colour, which was previously used for the discontinued $25 note. The $10000 note changed from green to red to avoid confusion with the green $500 note—this distinction was needed because the $10000 note was now smaller and closer in size to the $500. In the Portrait series, the $100 changed again—this time to an orange-yellow shade instead of brown, while the $10000 note was re-coloured gold to reflect its status as the highest denomination. With these changes, the only denominations today that retained their original colours are: $2, $5, $10, and $1000. Commemorative notes don’t follow these colour rules as they’re meant as collectibles and not for daily use.