Not a bad effort, rather then add the outline, add the outer bicep or the brachialis.

Once done, add the outline, start dark and shift the colour slightly, I’d say red to fit the orange yellow colour themes, then shift to a second outline and follow suite except with a lighter orangey colour, keep in mind these should still be dark.

Then your tricep, the tricep is like a horseshoe, I have yet to figure out how to make it well, try what you can I guess.