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Therapy-induced acquired resistance limits the clinical effectiveness of mutation-specific KRAS inhibitors in colorectal cancer (CRC). Here, we investigated whether broad-spectrum, active-state RAS inhibitors meet similar limitations. We found that KRAS-mutant CRC cell lines were sensitive to the RAS(ON) multiselective RAS inhibitor RMC-7977, given that treatment resulted in RAS-RAF-MEK-ERK pathway inhibition; halted proliferation; and, in some cases, induced apoptosis. RMC-7977 initially reduced the activity of a compartment-specific, dual-color reporter of ERK activity, with reporter reactivation emerging after long-term dose escalation. These drug-resistant cell populations exhibited distinct patterns of phospho-protein abundance, transcriptional activities, and genomic mutations, including a Y71H mutation in KRAS and an S257L mutation in RAF1. Transgenic expression of KRAS^{G13D, Y71H} or RAF1^S257L in drug-sensitive CRC cells induced resistance to RMC-7977. CRC cells that were resistant to RMC-7977 and harboring RAF1^S257L exhibited synergistic sensitivity to concurrent inhibition of RAS and RAF. Our findings demonstrate the power of reporter-assisted screening together with single-cell analyses for dissecting the complex landscape of therapy resistance. The strategy offers opportunities to develop clinically relevant combinatorial treatments to counteract the emergence of resistant cancer cells.