So at its most basic level, you're right.

If we take:

1 blue marble for tank gear
1 green marble for healer gear
5 red marbles for dps gear

There is indeed a bigger chance for a red marble to drop than a blue or green one.

However, a crucial detail here is that not all red marbles are the same color red. There isn't one universal "dps gear". There are 5 different kinds that are each only suitable for 1 or 2 jobs and as useless to the other dps jobs as they are to tanks and healers.

So adding that to our calculation we have:

1 blue marble for fending gear
1 green marble for healing gear
1 blood red marble for maiming gear
1 rolanberry red marble for casting gear
1 wine red marble for aiming gear
1 rust red marble for scouting gear
1 dalamud red marble for striking gear

Now, because we have 7 different colors, there is a 1/7 chance for every color to drop. Sure, 5 of those 7 might be a variety of red, but they're still not the same color. The chance for, say, wine red to drop is still 1/7, and for someone who needs wine red, rolanberry red is still a completely useless drop even though it's also red.