Dx with cellulities in shin. Prescription bactrim (sulfamethoxazole and trimethoprim) ds and keflex. Am elevating leg. Now leg is turning from warm and very bright-reddish to brownish/blackish?

Depends. This truly depends on exactly what it looks like. When cellulitis "burns out" with treatment, there is sometimes a brownish discoloration left behind. However, brown/black always raises the concern of necrosis/gangrene, so if there are worsening symptoms such as pain or swelling, i would immediately go to the er.
Skin color changes. As injured skin heals, the body creates inflammation to do so which means fresh blood goes to the area and with it, many kinds of healing factors and cells. One type is called melanocytes which carry our skin pigment melanin. A healing occurs, these cells get deposited int he area and give it the brownish coloration you see. Over time, there will be some fading as the injury matures.
Here are some ... What you as a 32-yr man is less likely related with poor circulation but needs to look for insect-bite, local injury, DVT, etc. Nonetheless, go to ER for urgent attention so to figure out what can be done to avoid worsening if possible. Of course, bring Hx of onset, degree, duration, evolution, & progress of all symptoms over time for analysis + reassessment + physicals + tests ... Best wish...