Colors are cultural, and there is no universally accepted range of colors that means "Start" and "Finish". (See http://www.informationisbeautiful.net/visualizations/colours-in-cultures/ for a look at the meanings people in different cultures ascribe to different colors.) That means even if you progress a color bar from red through amber to green, it may be valid only in Western cultures.
Alexey's answer regarding a progress bar is a potential solution. You could also combine that progress with a color shift, so the smaller portion of the bar (<25%) is red, the middle portion of the bar shifts to amber around 50%, then progresses towards green as the user nears 100%. You could even increase the saturation as the user nears completion, providing higher contrast the closer they get to their goal. This may meet your goal of providing color to people who want it, while still handing the visually impaired (Red/Green color blindness impacts something like 8% of males.)
Personally, I like work tasks to have text labels, and integrate into the navigation. For example, online stores often have a list of steps across the top with arrows showing the direction of activity, with the current step highlighted somehow:
Cart -> Buyer Info -> Shipping Info -> Payment
The paradigm is almost identical to tabs, but the presentation makes people think of flow. The user clearly knows what they've already done, where they are right now in the process, and what's left for them to do. If they want to go back to a previous step, they click on it. If they skip ahead (which you might choose to prevent) they still have an indication of what remains unfinished.