You can reach it through the west entrance of Glacier. Apgar is a great little village to go to, it has beach access and lots of cute little shops.
The multi color lake is named Lake McDonald. It has several access points including Apgar Visitor Center and Lake McDonald Lodge.
The best location is at Apgar, which requires a park pass, and is just passed the west entrance. McDonald Lodge is about 15 miles down the lake and is a very nice spot too, but you get a better view of the mountains and colored rocks at Apgar.
As everyone else has said, the lake is Lake Mcdonald. I recommend going to the Apgar portion, and if you want to play on the water try Glacier Outfitters, I had great luck with them. You can get there by going right in the west entrance, or going down the Going To The Sun Road from the east side.
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