I'm not sure if the comparison is between adult Simba and Mufasa or
cub Simba and Mufasa. Whatever the case is, I looked through some of MLK.com's screenshots to try to find two screenshots showing Mufasa and adult Simba during approximately the same time of day, when their colors would be the most accurate.
In this screenshot, cub Simba's fur is obviously lighter than Mufasa's. However, in
this screenshot, adult Simba's fur is darker, and when I compared his coat color to Mufasa's, I found that they were almost identical.
As for their colors appearing the same in various merchandise, I guess it was just a way of cutting corners. After all, the stuff is marketed toward kids, who probably aren't going to care if cub Simba's fur is the same color as Mufasa's.