What is your first volume about?
What do you want to tell your reader?
If the first volume is mainly action then do something thrilling.
Is the mystery slow pace? Then add an inviting neutral cover.
Is your volume a mystery right from the start? Then do something mysterious.
For a mystery comic I would go with either neutral or mysterious. Since it is volume 1 neutral works pretty well the other volumes can become mysterious.
Going high action, on the other hand, just attracts the wrong target audience which will probably not read your comic since they are looking for.... action.
Think of it like having a romance comic but the cover is horror. Your cover will only attract horror readers but they will not read the romance meanwhile all romance readers will avoid the horror cover.