This is my first post. Please be gentle. 
How do I transform a into b?
library(tidyverse)
a <- tribble(
~col, ~yr,
"red", 2010,
"red", 2012,
"blue", 2007,
"red", 2011,
"blue", 2010,
"red", 2007
)
b <- tribble(
~col, ~yr_min, ~yr_max,
"red", 2007, 2012,
"blue", 2007, 2010
)