You're just doing 2018 now? Where have you been? And how could you manage to miss the 1095-A? I got two paper copies by mail (several weeks apart) plus about a dozen emails, all of them marked IMPORTANT in all-caps, sometimes more than once!
The theory of the lower bound of 100% FPL for PTC and APTC (and cost-sharing reduction) was that Medicaid, with expansion if necessary, or SCHIP where applicable, should cover below that, although I don't know whether that worked for Colorado specifically. You were supposed to go back and notify the 'marketplace' when you realized your income would be different than estimated, and they would have automatically enrolled you in Medicaid if possible; obviously it's too late for that now. (BTW the ratios of 100% FPL and 400% FPL are the same everywhere; the FPL amount varies by year and household size, and also differs for Alaska and Hawaii but is the same for all other states, plus DC).
Anyway, if you 'received' excess APTC (that is, it was paid on your behalf to the insurer) -- and if you didn't qualify, then all of it was excess -- the amount you are required to repay depends on your MAGI-as-percent-of-FPL; see the form 8962 instructions at line 28. (Note only the most recent year is browsable and thus linkable on the website, so that is the 2019 version; the prior-year versions are available in PDF -- but this item didn't change.) Since you were under 100% FPL that limit is $300 if single or $600 otherwise.