"DM" -anti spam wrote:

1. It must be a tree that will not grow very tall.
2. It must have a crown that is not too dense, thus blocking light.
3. It must have foliage that changes colour through the seasons.
The garden faces due east, but in summer the sun would shine on the front of
the garden, where we will plant the tree, for most of the day.

It would help to know where on the planet earth you reside.

Bradford Pear is one tree that does well where we live in Zone 6. We
have three of them and they are a tree for all seasons. In the spring
they have flowers. In the summer they are a small beautiful tree. Then
in the fall they have fall foliage. Then in winter the small cherry
size pears are prized food for the squirrels. If you live in the ice
belt avoid the Bradford Pear. The ice will break it up.

We also have Bloodgood Japanese Maples. There are fairly easy to grow
and definitely have good foliage color with a strong tendancy toward the
red-greens turning more red. Grown in zones 5-8, red Japanese maples
reach a height of 30¹ with a spread of 20¹. Plant in full to partial sun
and well-drained soil. In zones 7-8 these fall foliage trees can profit
from the respite from the heat that they¹ll get if planted in an area
with partial sunlight.

Flowering crab apples are spectacular in bloom, although a few weeks
later than the Bradford pear, and have the added advantage (or
disadvantage, depending on your viewpoint) of bearing colorful fruit,
which can be ornamental in its own right, and which can provide food for
wildlife during the winter.

The native dogwood, Cornus florida, is a magnificent spring-flowering
tree, but it has fallen victim in our area to a fungal disease called
anthracnose. There is a good deal or work going on to develop resistant
varieties and hybrids. For now, Cornus kousa, the Kousa dogwood, is a
good substitute, and mature specimens display very attractive bark,
which extends the period of appreciation right through winter. There are
many named varieties: get a recommendation from someone you trust, or
try to see plants in flower over the next few months.

Another native, the Eastern redbud (Cercis canadensis), is also
available as a number of cultivars, including the burgundy-leafed
"Forest Pansy" and "Royal White," a variety which calls the whole
"redbud" thing into question. Redbuds are easily stressed, and will
develop cankers and fungal wilts in this weakened state - give them good
growing conditions and adequate water.
--
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Cheers, Steve Henning in Reading, PA USA Zone 6