jtpr wrote:
I am buying some new teak lawn furniture. I want it to remain the
color it is, not turn grey. Is there a treatment I can use to
accomplish this?

You can't get there from here.

The only way to treat teak if you want it to maintain the color is to
become a slave to your furniture.

Probably the best way is to wipe the surface with acetone to remove
surface oils, then coat with 4-6 coats of epoxy, also sanding lightly
between coats.

Allow to cure for about a week after the last coat then apply about
4-6 coats of marine varnish that has UV inhibitors in it.

Allow 24-48 hours between coats, again sanding lightly between coats.

Maintenance:

At the sign of the first scratch in the varnish, repair it or the UV
will damage the epoxy.

(I know people who keep a nail polish bottle of varnish handy to make
scratch repairs)

As an alternate to the above, learn to enjoy gray teak. It grows on you.

Lew