I have aover heating problem,I replaced the radiator and still the same problem.What is the two hard lines that are connected to the radiator,the fluid looks like hydraulic or brake fluid. Does this fluid have any effects on the cooling system?

I live in SoCal so there's endless stop and go driving around town, and on a hot day, when I'm waiting at a stop light, I'll watch that temperature gauge climb from the half way point where it normally sits all the way up to the 3/4 point. then I crank the heat and crack a window, on a 80 degree day. that coupled with faster moving traffic tends to bring it back down to normal. I guess it isn't technically OVER heating. but I'm worried regardless.

OK so this has been going on for awhile, obviously it occurs more often in the summer then in the winter. When I'm actually driving my car it runs fine but when I am idling at like a stop sign/light or stuck in traffic my car starts to overheat, but then once I get going again my car starts to cool down again and it's back to normal. I found out that it's not coolant and now I'm getting super frustrated! Please help!!