Color-color relations for red giants in star clusters

New Johnson-Cousins UBVRI photometry of giants in globular clusters is combined with JHK photometry on the CIT system to produce color sequences for giants from the globular clusters M3, M5, M13, and M92,. UBVRI data are also presented for giants in the metal-rich open cluster NGC 6791. These data fill a gap in the literature, especially for the R and I bands. We provide the empirical relations between broadband colors for various [Fe/H] values for metal-poor giants. The color sequences for U - B and B - V show clear separations for different [Fe/H] values. We also find weak, although unexpected, metallicity dependences of V - R, V - I, and J - K colors. H - K is metal insensitive. The above colors are plotted as a function of V-K, and a literature (V- K)-T-eff relation is given.