on Public Works projects in Colorado can General Contractor hold sub pay until conditional waiver for the payable amount is received?
Unless conditional lien waivers are specifically required (under your contract) before payment, then no - conditional waivers shouldn't be a mandtory prerequisite to getting paid. Granted, that doesn't mean that your customer won't threaten to withhold payment until a lien waiver is in hand - and that can turn into a standoff. In which case, it'd likely come down to a business decision when deciding whether to hold your ground and see what happens, or to relent and provide a conditional waiver, as requested.
Keep in mind, though - conditional waivers shouldn't waive any rights until payment is in hand. So, there might not be much risk in providing one, especially when working with a trusted or at least familiar customer. Levelset discusses why conditional waivers are used in these situations, here: Do I Have to Sign a Lien Waiver to Get Paid?
If you do decide to provide your customer with a lien waiver, Levelset has free downloadable CO lien waiver forms here: Free Colorado Lien Waiver Template & Forms. Additionally, this article will be useful for deciding which waiver forms to use: The Ultimate Guide to Lien Waivers in Construction.