Official image will not boot

[topsys]

HI All, thanks for any advice in advance.
I have new raspberry pi 3 B+
I have the official power supply for this unit
I have 2 SANDISK 16gb cards bought with preloaded noobs & Raspbian installed, both booting correctly
Downloaded the RetroPie 4.3 (release 21/09/17) for Raspberry Pi 2/3 from the official link
I Formatted one of the 16gb cards using SD Card Formatter
I imaged the card using Etcher
-- Result I get a red flashing power indicator and a rainbow effect screen image

I reformatted the sd card and tried imaging using win32 Disk Imager
--Again same result

I downloaded the original retro pie source image a second time and tried both stages above
-- Again same result as previously

Next I formatted the SD Cards and imaged with the official Raspbian OS image
Booted up 1st time

So my assumptions are:
SD card is good
Power supply is Good
Raspberry Pi 3 B+ Hardware is good

Can any one please offer any advice?
Many thanks

[rbaker]

@topsys https://retropie.org.uk/forum/topic/16307/retropie-4-3-15-stretch-images-for-testing