Red tips on acacia bipinnate leaves

A few weeks ago I acquired two small acacia melanoxylon (Australian blackwood) seedlings, both about 12" tall. They are placed on the west side of my house and get full sun from midday to sundown. I've noticed recently that the bipinnate leaves have developed a maroonish tint at their tips that fades out towards the axis. I had feared that this might be a result of too much sun exposure, but the new buds are also showing this tint, so I don't think the sun is burning the leaves. Is this symptomatic of any particular problem, or just a natural coloring as the tree begins to grow? I hope and suspect the latter.
I am located in central Florida, zone 9b.
Many thanks.
- Kevin
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