  We present an analysis of the intrinsic colors and temperatures of 5-30 Myr
old pre-main sequence (pre-MS) stars using the F0 through M9 type members of
nearby, negligibly reddened groups: Eta Cha cluster, TW Hydra Association, Beta
Pic Moving Group, and Tucana-Horologium Association. To check the consistency
of spectral types from the literature, we estimate new spectral types for 52
nearby pre-MS stars with spectral types F3 through M4 using optical spectra
taken with the SMARTS 1.5-m telescope. Combining these new types with published
spectral types, and photometry from the literature (Johnson-Cousins BVIc, 2MASS
JHKs and WISE W1, W2, W3, and W4), we derive a new empirical spectral
type-color sequence for 5-30 Myr old pre-MS stars. Colors for pre-MS stars
match dwarf colors for some spectral types and colors, but for other spectral
types and colors, deviations can exceed 0.3 mag. We estimate effective
temperatures (Teff) and bolometric corrections (BCs) for our pre-MS star sample
through comparing their photometry to synthetic photometry generated using the
BT-Settl grid of model atmosphere spectra. We derive a new Teff and BC scale
for pre-MS stars, which should be a more appropriate match for T Tauri stars
than often-adopted dwarf star scales. While our new Teff scale for pre-MS stars
is within ~100 K of dwarfs at a given spectral type for stars <G5, for G5
through K6, the pre-MS stars are ~250 K cooler than their main sequence
counterparts. Lastly, we present (1) a modern Teff, optical/IR color, and
bolometric correction sequence for O9V-M9V MS stars based on an extensive
literature survey, (2) a revised Q-method relation for dereddening UBV
photometry of OB-type stars, and (3) introduce two candidate spectral standard
stars as representatives of spectral types K8V and K9V.
