In general, you should wait for 6-7 nodes before topping. This is just to avoid a low-probability of shocking a young plant. I've toppped early without issues, but in a larger sample size across many years, it's a bad idea to top early.
Beside that, when you top should be part of a greater plan for canopy management. You only need 2-4 colas per sq ft to maximize yield (grams per m^2 or ft^2 etc). More doesn't help. More just causes additional efforts of pruning and training to avoid disease vectors and the mass of flower just gets spread out more so you also get a harder trim for all that extra effort.
That early single bladed leaf yellowing isn't too serious. Still a sign it's not getting quite enough nitrogen, but probably not a severe degree of deficiency. The plant will shed that early leaf easier than others.
I'd wait and see in this case... if the paling does not progress up the plant, all is fine.
So, have a target for your canopy. Formulate a systematic plant to train your plant to get there in the most efficient way. If it's 4 sq ft, you want a total of ~10-12 colas divided by number of plants. From there figure out easiest way to top or LST the plant to get that many primary branches. Keep them level as they grow -- manipulate apical dominance temporarily to help some catch up or slow others down -- highest branch takes the most resources (simplified... a branch of the apical meristem is favored over secondary branching etc... but you should be able to recognize and quickly learn what gets favored and how to manipulate it - simple stuff)
anecdote - I want 10-12 colas per plant. Once it gets the 6th node is formed and the growth tip is above that, I top above the 4th node. I prune off the bottom pair of axilla, but not the leaves. Always keep the leaves. So, i have 6 branches that will result. I train them to even out growth and top them all for 12 primary branches.
There's genetic variety.. some plants just don't grow like other plants. So, when i have a laggard branch that won't keep up, i top all but that branch in hopes it can catch up feedingn 1 mouth instead of 2 off that primary branch... smaller plants might end up with 9-10 colas due to adapting to the reality that sits in front of you. Can't force a square peg through a round hole, when L&W = diameter.
The key is understand yield is more about surface of canopy and light capture than more colas. At some point, more colas just spreads the mass out more and gives you a higher leaf:bud ratio. 2-3 colas per sq ft, and maybe 4 colas per sq ft in the case of a shitty pheno with a genetic trait of really tiny buds, is all you need.
Last year i did 2 colas/sq ft and this year it's closer to 2.75 and the yield is no different. Slightly smaller, more spread out buds, which is actually what i wanted in this case to help create consistency in drying and avoid the overly large buds that also come with elevated risk of mold developing -- but still easily trimmed buds with as minimal leaf ratio as that the gentics allow.
it's a "happy zone" not "min/max" strategy.