Ken Cox's Choice. Best autumn colour is achieved in dry crisp autumns. Mild wet ones are less impressive. Most of those listed below will colour well every year.
Autumn colour is caused by sugars in the leaves which turn to red, orange and yellow as the green chlorophyll in the leaves starts to fade and the leaves die and dry up.
Acer palmatum (Japanese maple), incl. A. palmatum ‘Osakasuki’ (probably the best of all)
Berberis thunbergii
Betula (birch)
Cotinus ‘Grace’ 
Enkianthus
Euonymus alatus, E. europaeus ‘Red Cascade’
Fothergilla
Larix
Parrotia
Parthenocissus
Rhododendron (azalea) luteum and other deciduous azaleas
Rhus typhina
Saskatoon (Amelanchier)
Some of the other trees and shrubs famed for Autumn colour dont always colour well in Scotland due to the lack of crisp dry Autumn weather: Liquidambar, Liriodendron, can be good or disappointing depending on the year.
