I would agree that BSF larvae is an indication of a cold compost operation.

If one could tell the difference between the larva of the various fly species then maybe, possibly having the larva of the Black Soldier Fly might be beneficial. However, since the larva of the Common House Fly (known to spread disease pathogens), the Stable Fly, and many other species of flies that are pests look very similar to most of us then having maggots in your compost is not a good thing. All of these maggots need an environment that is too moist for good, aerobic composting.

Here is a link that might be useful: What UI says about maggots

Its easy to tell the difference between house fly larvae and black soldier fly larvae.

BSF Larva:
Black soldier fly larvae grow up to 2cm long, and are flattened dorso-ventrally with a headlike narrow anterior structure bearing lateral eye spots and visible antennae and maxillae.

The larvae are clearly segmented and slightly tapered. They are dull tan to white in color, but often appear darker brown because of the adherence of manure (especially in poultry houses).

House fly larvae:
The integument of the house fly larva consists of an outer acellular cuticle and an inner single layer of epithelium which rests on a basement membrane. The cuticle is covered with an epicuticle and has a stratified structure. The cuticle is 5µm thick in young larvae (36 hours old), 25µm in 60-hour-old larvae and 40µm in last (third) instar larvae.

The larva is white and cylindrical, with the posterior end broad and flattened. It tapers anteriorly.
There are no eyes or appendages, although there are some ventral spiny ridges which aid locomotion. The larvae have 13 segments, but the first two are partially fused so that only 12 segments are apparent.

Some of the internal organs may be seen through the cuticle. The spiracles are openings for air to enter the respiratory system of the larva. The posterior spiracles (on the broad blunt end) are distinctive.

Here is a link that might be useful: Fly information

If your fly larva are eating manure they will be brownish in color, but unless your compost pile has manure on the top (and manure should never be the top of a compost pile) the maggots in any compost pile will be the creamy color most people see. Even the common house fly larva that eat manure will be brownish in color.

I wouldn't rely on color to identify larvae either.

I would use size, shape and other indicators described previously (visible antennae and eye spots). BSF Larvae are larger and flatter. House Fly larvae are cylindrical, rounder, and smaller.