I played colossus and liked it a lot. The one thing it does not lack is damage, I would say the highest damage javelins are interceptor and colossus followed by storm and ranger. With the colossus you can both create and detonate your own combos pretty much constantly (with lightning coil + flamethrower) for massive damage, the ultimate ability also does the highest amount of aoe damage while the interceptor ultimate does the highest amount of single target damage.
The one thing to keep in mind with the colossus is that it's a very different playstyle compared to the ranger (and probably the storm). As a colossus you're not that useful at range, and ducking behind cover is useless as you're not regenerating any shields or health. You're also not a tank unlike what lots of people think. You can't just wade into battle and stand there taking all the hits, that'll kill you quickly.
What you are is basically a brawler. Aim for the nearest enemy (or rather pack of enemies), use the shield and slam into them. The colossus components give you way more health and you basically need those to be effective. The health bar is your buffer while you're in the thick of action. You have a lot of explosive combo detonators and atleast the lightning coil can be used while using the shield so you can both block all attacks and detonate combos around you at the same time.
Your guns are basically secondary and only really used if you need to hit a weakspot or something like that or if you're stuck at range for whatever reason, your abilities are your main attacks coupled with shield + melee when you need to knock something down