recently replaced 4 new plugs on my carb engine - correct plugs specs based on the service manual. after running for about 50kms saw the plugs' color are not the same:

plugs no. 1 & 3 are very dark, carbonized
plugs no. 2 & 4 light to dark brown. - looks normal

we checked the compression pressure but cylinders are nearly equal above 155psi.

what could possibly be wrong? what should i check next?
is valve clearance related?

thanks in advance for your ideas or suggestions