Mass Effect presents a somewhat unorthodox take on the traditional “good or evil” dichotomy in many video games, having instead a “paragon or renegade” system. First, it’s not a strict dichotomy at all - you don’t have to pick just one, and while the two options are often presented in an either/or context, picking one doesn’t mean you diminish the other. Instead they should be viewed as… different toolsets Shepard can use as the situation demands.
Renegade isn’t strictly evil and Paragon isn’t wholly good, although the latter is closer to the standard convention of “good” than the former is to “evil”. The Renegade is pragmatic, yes, even to the point of being ruthless, nonconformist (sometimes even verging on anarchistic) and irreverent of authority. They do what needs to be done and seize opportunities as they present themselves, even if they can be brutal, vindictive, and at times, even cruel. The Paragon, on the other hand is selfless, honorable, merciful… and honestly a bit naive. The Renegade’s brash initiative and the Paragon’s soothing diplomacy are both skills that can serve you well, depending on the situation.
In the first Mass Effect game, you’re more free to pursue a hybrid approach, especially on multiple playthroughs. Earning Paragon/Renegade points will, at various plateaus, earn you free ranks in Charm and Intimidate, which are used to determine the difficulty threshold of Paragon and Renegade checks you can select during dialog. These choices will be colored red (for Intimidate) or blue (for Paragon). Otherwise it’s generally safe to assume that options on the top right are usually Renegade and options on the bottom right tend to be Paragon (options to the left tend not to have a Paragon/Renegade bias).
There are plenty of Paragon/Renegade points to go around in the game; if you just want to do a single playthrough as a (mostly) pure Paragon/Renegade it should be a simple enough matter to put points into Charm/Intimidate when you can and pick the blue/red, top/bottom options during dialog, as you prefer. If you’re playing Renegade, just consider exercising some restraint when it comes to your companions, should you want to romance them, and don’t decline assignments just because the option may be on the bottom.
In later games, the points available are fewer and there are no free rides, so you’ll have to focus on one if you wish to max it out… but there’s still some opportunity for fluidity here and there.