Muriatic acid is the last thing you want near stainless, even fairly weak commercial brick cleaning acids can result in a right mess- staining to pitting corrosion. Stainless steel gets its corrosion resistance from a chromium rich oxide surface layer, reducing acids lack the oxidising properties that are needed for stainless to maintain it's passive oxide layer
Pickling paste hasn't been banned, it's not something to use unless you have to though, typically a mixture of nitric and hydrofluoric acid the fumes are nasty and well, that's the nicest thing to say about it! For light staining and iron contamination citric, phosphoric and oxalic acids can be used but they will typically be very slow at removing heat tint by themselves- the process can be sped up by using them as the electrolyte in electrochemical weld cleaning and passivation
As said getting them ceramic coated is the only way to keep them from getting discoloured (well, unless you never drive it after fitting 'em). Heat tint colours start forming on stainless at a bit under 300*C, pale yellow then straw getting darker to browns at around 400* then purplish reds followed by blues at 550ish. The pretty colours are the result of a thickened oxide layer that refracts light differently
If they're non-magnetic (austenitic stainless) they're probably 304L (316L is a possability as 'hygienic bends' are a cheaper source for those without their own mandrel bending kit). OE manufacturers favour 409 (a ferritic stainless) which is strongly magnetic
