How do you get dingy windows clean if its double pain glass?

Most likely the seal is broken on the two panes. The moisture collects in between . Consult a professional.

If the seal is broken and there is a frosty appearance, it won't ever get clean because it's between the panes. The moisture and salts in a broken seal are there to stay. But, if the seal is good and the exterior surfaces are just dirty, here's my favorite glass cleaner. Make a thin paste of Bon Ami cleanser with water. Rub the paste on the glass surface with a clean clothes like an old T shirt. Let the mixture dry to a white haze for about 20 minutes. Buff off with another clean clothes. Change cloths frequently. This mixture is great for car windows and mirrors and gets off that stubborn smoke film.

If the discolor is in between the panes there is no solution I know of.

Windows need to be resealed.