Hello,
On the speedtree forum, i read it might be due to the specular map:
Be careful with specular. UE4’s use of specular doesn’t completely jive with the specular maps SpeedTree uses, so sometimes it can make leaves glow in some lighting situations. But other than the that, I am unsure about what could cause the trees to be so bright. Did you build lighting on them?
The color variation per tree is caused by the color variation node in the leaf and billboard materials. The default value should be very low, but you can edit it, or remove that node entirely.