I just unboxed a Pimoroni Raspberry Pi 3 kit, and everything’s fine, with one exception: it’s hooked up to an old computer monitor, and there are no red pixels on the screen, The raspberry is blue-green, for example. It’s not a fault with the monitor, which is perfectly able to display red. I’m assuming there’s a fault either with the VGA cable, or with the HDMI adapter (I’m using one of the basic little ones sold by Pimoroni).
I am assuming that replacing either of these parts will solve the problem, but I am wondering whether there is anything else I should be trying before I do that?
I don’t think there is, no. If possible I’d try the Pi straight into your TV if it has an HDMI input, just to be sure it’s either the adapter or cable, but I’d say it’s very likely.
It might be worth re-seating the cable or giving it a wiggle to see if anything changes.
Also try reversing the VGA cable - it should have a female connector at each end, right? If you reverse it by swapping the ends between the monitor and adaptor, and it has a bad connection, that connection will move from the Red channel to a non-essential Ground connection.