If its one thing people will never ever seem to grasp its that just because the special coolant is the same color doesn’t mean it will work for long.
Most the fun colored coolants now are that color as a warning or “hello, I’m not your normal green stuff” Take VW/Audi’s: Since the late 80s the switched to G11 blue, then mid 90s to G12 pink. Sure the green stuff will work, but god forbid you didn’t clean the system out really well…..it turns to mud. And trust me, Ive flushed enough old cooling systems out of pudding because coolants were mixed, Even slightly.
A pro cleaning kit will make life easier, but it can be done on your own in the drive way too. There are kits available that you basically splice in a fitting to the lower and upper rad hose. The lower has a petcock on it, the upper has a fitting for a garden hose. This makes the rinse cycle easier. I’ve never tried the fittings but know guys with old trucks that they left them in for good and worked well.
For mildly dirty systems I would drain the entire system, remove the T stat, and then run water in through the top of the block letting it pour out the lower rad hose. Do the rad the same way, and the heat core. Get as much crap out as possible. After that install a new T stat and fill it with just water and bleed it out properly. Drive it around for a few days letting it run and full operating temp so you know the T stat is opening and coolant is flowing through the rad. Now is when you’ll see if some one used stop leak. After a few days, drain it all out again, flush it out again, and fill it with your coolant mix of choice. I try to stick to what came with the car.
If the system is as dirty as you say it is you may want to put some effort into it. I’ve used this trick a few times and it works great! Got it from a truck mechanic. The drain, flush and refill with water is the same, but add a cup of cascade dishwasher powder to the mix. That stuff will dissolve a lot of junk, and clean the surfaces inside really well. Drive it for a few days like above, and then drain, flush again. Then fill it with just water, and leave that in for a few days of driving to rinse it out. Then drain, flush again. Now fill with coolant. If you have those nifty hose fittings for easier flush you can make quick work of that. If the cascade trick was dodgy or dangerous I wouldn’t have used it in my own car a couple years back.
In really bad cases you may find after a few months your new coolant is murkey again. A simple drain and refill fixes that.
I mentioned “I” I would do the T stat at the same time. A temp gauge should be in the middle between hot and cold when its at operating temp. If your’s is reading cold it may be gummed up with junk.