I wouldn't presume to tell you where to get coloured in, but I guess it might be worth considering your desired audience (lets not pretend Tat's aren't an exercise in vanity)
Lots of people will see a forearm, perhaps fewer will see a calf, and less likely up close, a left cheek...well I don't know your lifestyle, but that'll probably get seen only on special occasions by a limited number of people

As for nuance in the design - I'm always a fan of layers or "hidden" meanings - they don't have to be particularly special (in fact, when you explain them, they're always a bit "meh" ) - but for example, I once owned a fairly unreliable pushbike that had a lot of 15mm nuts on it, so I kept a 15mm spanner on hand at all times, so (were I to get the same tat) i might go for that size....
The 10mm socket joke is...well it is what it is.
I'm sure at some point you've lost a tool, found a tool, thrown a tool, or have some kind of personalised memory or in-joke that you can apply.
It's not much, but it might feel better than randomly picking a number

Were it me - I would not do a ratchet & a wrench, as they are fundamentally the same thing, and it might be nice to have some....I'm not sure juxtaposition is the word I'm looking for? but something like that.
Is there some other reference for mechanic-ing you like? Spark plugs? Feeler gauges? 32 oz hammer? heaven forbid, a workshop manual?