The most likely candidate is INVASION FROM 2500 by Norman Edwards and published by Monarch Books in 1964. And, yes, there is a glowing arch with a line of futuristic tanks emerging. The novel has the following apt dedication: "To Terry Carr and Ted White, who made this book possible". Terry Carr and Ted White collaborated under the pseudonym of "Norman Edwards", so they certainly made the book possible. Your memory had added the fancy details of rainbow colors. The arch on the cover is a golden color and surrounded by a bright white glow.
The blurb on the back cover has this evocative quotation:
"Jack stopped the car and ran toward the light. He was greeted by a fantastic sight: a huge, weirdly glowing arch, through which were pouring strange-looking tanks, airships and metallic-clad soldiers carrying sinister weapons."
Hope this helps.