Ruger's official name is the "Single Six Lightweight Model Revolver". All other names are collector nicknames. All lightweights are not tri colors; some are all blue and only the earliest had flat gates. So the factory name does not distinguish if it's a tri color or not, hence the need for collector nick names.
Your # is from 1957. However, if it has an S stamped on it in front of the serial # or bottom of frame in front of the trigger guard it's a 'second' that could be from 8/1964 or 4/1965. The seconds can have a blued steel cyl and those of course are not literally tri colors, but two tones.
Six variations and more sub-variations are identified by collectors, so besides the flat gate you identified on yours, we'd need to know other things, some of which Ale mentioned above; the style of the ejector rod button (flat serrated or round), ejector rod housing material Alum alloy or steel, etc.