In photometry, we determine the color index (or colour index if you're from my part of the world) as the difference in magnitudes between two wavelength filters. The reason that we use the difference, is because the magnitude system is logarithmic, so using the difference tells us the ratio of the intensities of the two wavelength bands.
To determine the B-V index, you'd need the magnitudes in the B band (which is centered on $\lambda = 445$nm) and in the V band, centered on $\lambda = 551$nm.
To find the U-B index, you'd also need the U-magnitude, at $\lambda = 365$nm.
So, as @zephyr said, there's no way to convert between the two color indices.