After a report is finished, I am writing a letter commending a person.

Should it be...

"His teammates often ask him for advice on this project."
or
"His colleagues often asked him for advice on this project."

ask or asked?

Since the project appears to be finished, you'd use the past tense -- "asked." If the project hasn't been completed, the present tense is correct.

I prefer the use of colleagues, rather than teammates in this context.

Would it be better if "His colleagues often went to him for advice."? or is there any better way to phrase it?

Another way to phrase it:

His colleagues often asked his advice on this project.

To determine the correct verb tense for your sentence, you need to consider the context and the timeframe in which the action is taking place. Here's how you can decide between "ask" and "asked":

1. Current Situation: If you are referring to a present or ongoing situation, where the person is still providing advice to their teammates, you should use the present tense. In this case, you would use "ask" to indicate that the person's teammates continue to seek advice.

Example: "His teammates often ask him for advice on this project."

2. Past Action: If you are referring to a situation that happened in the past and is already completed, you should use the past tense. In this case, you would use "asked" to indicate that the person's colleagues asked for advice on the project in the past.

Example: "His colleagues often asked him for advice on this project."

Consider the timeline of the events and choose the appropriate verb tense based on whether the action is ongoing or completed.