A shirt is a lightweight covering for the upper torso and arms. This article is sometimes referred to as an undertunic although this appears to be modern rather than period terminology. Typically made of linen right throughout the medieval period and beyond. Neck styles and sleeves dimensions are quite variable and the length is typically longer than modern shirts falling to the mid thigh.
If a tunic was worn then typically the shirt would not extend beyond the hem of the outer garment. This became less of a rule as the outer garmet shortened and by the 15th century the shirt could be significantly longer than the doublet.
Typically the shirt was white or off-white in colour.