I use the R45S Spark plugs at my 1968 Le Mans (400cui engine from 1970, heads: 62 from 1968, tri-power carb, Performance cam, unknown...)
I think, the spark plugs are a little too bright, and I want to use colder spark plugs, which should I buy ?
R44S, R43S ? other brand ?
Hmmmm. A little too "bright?" That's a new description to me.
Today's blend of gas does not produce the coloring of old. Very difficult to read plugs, so you can't really judge by the "brightness." You can go colder, but then you want to make sure you are not fouling the plug and not getting a good burn out of them.
Rather than go colder, why not go a little more on the fuel? Today's gas runs leaner that in the past, especially with the ethanol if you are using it. And a bigger cam may require more fuel as well. Factory jetting is .062 in the center carb and .068 on the outboard carbs. Jet sizes for best performance is .073 in the center and .069 on the outboard carbs. This is what was done by Milt Schornack with their Royal Bobcat package on the 1966 tri-power cars having the "ram air" hood package.
If you are not experiencing any issues like "pining" or "detonation," I would leave them alone. The R44S, as I recall another member was asking about this, are no longer made and were very expensive on Ebay. I assume there are other brands that can be cross referenced. The Champion is J12 series works and colder plug used for maximum performance at the track would be the J10.
Stay away from split fires, or the "thin" wire type plugs. Stick with a conventional type electrode.
Keep the plug gap .032"- .035" as it takes less energy to jump the gap and you get a stronger spark.