What does it indicate?

#2 and #3 plugs are tan all around
#1 and #4 plugs are white on the half next to the gap

white is overheated - lean fuel mix or to much advance on ignition timing?

2 and 3 would seem normal

White is correct. But in this case, they are perfect tan over one-half the insulator.

Timing is 10 degree BTDC, very conservative.

Carburetor is stock Datsun 1200 carburetor.

i think the plugs are 1 heat range too hot? perhaps you could go 1 range colder. the AFR looks a little lean though.

But two plugs are perfect color. Will not richening the AFR make them too rich?

if it is my engine, i would change the plug 1 range colder, and drive it for a while. after a while, open the plug and see if the ratio is good or lean. my previous experience, if i were using 1 heat range too hot, the plug reading shows the mixture a little too lean, but everything else is perfect. drivability, cold start (in malaysia where everyday is summer).

have you done compression test?

Yes, compression is good. Plugs show no sign of running lean -- every plug is a nice tan colour. They don't look hot to me, no overheating or blistering is evident. The plugs that are part white are also part Tan! Look at the photos carefully to see what I mean.

Apparently this half/half plug condition is not uncommon. Searching, I see it is reported from Nissan to Chevrolet and from chain saws to snowmobiles. No one seems to know what is is, but people are guessing at the cause -- all different sorts of guesses.

yeah they look pretty good,my plugs in my daily kept doing that but worse ,almost sure its a bi product of all the burnt fuel additives.

and use denso or ngk plugs. there better than most. Champion make good mower plugs but thats it. ive had several dead out of the box (platinum and iridium included)
refuse to use them in customers cars these days.

I agree they`re running well but I prefer Jp plugs in Jp donks
what was your economy by the end?