At 265F, sugar is at the top end of the 'hard-ball' stage. It is still clear. There are two more stages left to go (in candymaking terms)--'soft-crack' (270-290F) and 'hard-crack' (300-310F)--before sugar will start to color and caramelize (320F). It burns above 350F.
The rub is usually the salient issue in deep coloring. Sugar melts and becomes quite sticky (to which smoke particulates readily adhere), chilies and herbs darken and, if glazes and or sauces are applied, further color is added and this color deepens as evaporation of moisture occurs and the glaze or sauce concentrates. All this, combined with the coloring that occurs due to Maillard reactions at the meat's surface contribute to the final depth and darkness of the color.
Perhaps surprising to many, cooktemp doesn't have much to do with it--as long as that temp is within reason. These spares were rubbed with a high sugar/low chile rub that was also light on the herbs. They smoked a 325 then were foiled and cooked till tender at ~365, unfoiled, thinly glazed with a pineapple-stock-honey-Dijon glaze and returned to the smoker for a few minutes to set the glaze.
These backs were cooked at slightly higher temps in a similar fashion but with a low sugar/high chile rub, with substantial herb components, and finished with a very lightly colored glaze of calamondin liqueur, butter and pomegranate.
Similar cooktemps but markedly different rub components between the two.