I only have four trees in my entire garden; with space for only a few trees I figured I'd best choose ones with the most impact. So I went for all colored foliage trees, and since I planned the garden for summer and autumn (another strategy for tiny gardens), all the trees are deciduous except for yew and bamboo hedging.
I started by planting a weeping willowleaf pear (Pyrus salicifolia 'Pendula') for its foliage that is so silvery it's often mistaken for an olive tree. I love its umbrella-like weeping shape and how this little tree appears both dense and light, hardly casting a shadow in the landscape.
I fell for the absolutely gorgeous foliage of the chocolate mimosa tree (Alibizia julibrissin 'Summer Chocolate') even though it'd prefer growing in a warmer climate. I planted it in a half barrel, its base fluffed out by amber carexes and orange-blooming Begonia boliviensis. This little tree is a stunner; plant it in the warmest spot in your garden, and don't panic when it doesn't leaf out until mid-June. The deep purple ferny foliage is worth the wait.
Unfortunately, my only two other trees and the centerpieces of the garden are also late to leaf out...leaving the garden looking pretty bare well into May. The burgundy-leafed redbud (Cercis canadensis 'Forest Pansy') has a spreading shape with large, heart-shaped, cordovan-colored leaves that glow when the light shines through them. This beauty holds pride of place in the center of the garden and well deserves it - have you ever seen such gorgeous leaves?
The last tree is a golden locust (Robinia pseudoacacia 'Frisia'), which is suffering dieback from last winter's sudden December freeze, but has leafed out in its usual stunning, shining gold. The foliage comes on this brilliant color and holds until the leaves fall late in the season. I can't imagine another tree that warms and brightens a garden like a golden locust; I have it planted to fill out a corner of the garden, where a purple Clematis roguchi clambers up, its nodding little purple bell flowers contrasting with the gold in a combination unfortunately and unintentional Huskie-esque.