This is really a matter of taste: At the moment, we try to reuse as many sprites as we can (e.g. recoloring the lemmings sprites for climbers, zombies, ... as well), because this ensures that players will always see a consistent look. Otherwise one might forget to change some types of the sprites and suddenly when you do a certain thing in-game, you will be presented with a totally different sprite.
In short: While having separate sprites adds more flexibility, is is also more error-prone.
And if I remember correctly, we implemented the recoloring of the time digits in a halfway sensible way by exchanging the red and green values of the RGB-representation of the sprite, instead of just replacing a few fixed colors by others. So there is actually a pretty decent chance, that replacement digits don't look too bad. But please don't blame me if I misremembered this point