High-temperature superconducting <inline-formula><tex-math notation="LaTeX">\rm{REBa}_{2} Cu_{3} O_{7-x}</tex-math></inline-formula> ( REBCO ) conductors have the potential to generate a high magnetic field over a broad temperature range. The corresponding accelerator magnet technology, still in its infancy, can be attractive for future energy-frontier particle colliders such as a multi-TeV muon collider. To help develop the technology, we explore the requirements and potential characteristics of a REBCO magnet, operating at 4.2 or 20 K, with a dipole field of 8 - 10 T in a clear aperture of 150 mm. We use the canted <inline-formula><tex-math notation="LaTeX">\cos \theta</tex-math></inline-formula> magnet configuration to reduce the electromagnetic stresses on the conductors. We present the resulting dipole fields, field gradients for combined-function cases, conductor stresses, magnet dimensions and conductor lengths. We also discuss the conductor performance that is required to achieve the target dipole field at 4.2 and 20 K. The information can provide useful input to the development of REBCO magnet and conductor technology for collider-ring magnets in a muon collider.