They all have specific uses and limitations. For most general gunsmith applications the blue is the defalt. Hand removal and will cover from small to large fasteners. Red is the higher strength and requires heat for removal.
For non threaded the 609 compound is the defalt- the 620 is for high heat application.
Be very aware of what some of the variants do and are ment to do or you can get in trouble. This product is also MEK based and exposure can be deadly.
In my day job I use these products by the pint- I have seen the good and the bad.
al
