cast iron ??

started using cast iron skillets a few months ago. I have a 12inch and a 10. Both older not sure of make or year. The one has seasoned up nicely turning jet black in color. The other one is staying kind of like a brownish color and not turning black like the other. Anyone know why this might be happening? thanks for your input.

Corey what happens with seasoning is the hydrocarbon coating (oil) breaks down , the hydrogen boils off, leaving a thin coat of carbon, which protects & is slick.
Any oil will work ( use food safe)
The animal fats with low smoke points may degrade easier,I usually use bacon grease for pans & canola spray for grills cause I've got it & it's easy to apply the spray in a grill.as the process continues the color changes from an almost bronze color toward black. It may take a number of cooks to get the darker color.

thanks for the reply john.