The colours could be analog, continuously changing with pressure instead of discrete, although a solid or flashing red when below minimum pressure would be best. However, you would have to train your eye for the colours that are important to you.
Bikes without an ammeter hole in the headlamp shell would need an accessory bracket for an analog gauge and would look less stock but if acceptable it can be done.
The stepper motors used in GM vehicles for the gauges are small enough to fit in the ammeter hole.
The advantage of a light over a gauge is that you do not have to look directly at it to notice a change if it is in your peripheral vision.
Unless there is a problem with the charging system, an analog volt meter is not very useful. Just knowing whether the battery
is charging or not is important to keep the battery
topped up, especially with an electric starter. A gauge can change to alternately display voltage and pressure with a light to show which is being displayed or the gauge could be illuminated with a coloured light showing battery