How to group a vector into a list of vectors?

split.data.frame is a good way to organize this; then extract the color component.

d <- data.frame(dressId=c(6,9,10,10,10,12,12),
               color=factor(c("yellow","red","green",
                              "purple","yellow",
                              "purple","red"),
                 levels=c("red","orange","yellow",
                          "green","blue","purple")))

I think the version you want is actually this:

ss <- split.data.frame(d,d$dressId)

You can get something more like the list you requested by extracting the color component:

lapply(ss,"[[","color")

In addition to split, you should consider aggregate. Use c or I as the aggregation function to get your list column:

out <- aggregate(color ~ dressId, mydf, c)
out
#   dressId                 color
# 1       6                yellow
# 2       9                   red
# 3      10 green, purple, yellow
# 4      12           purple, red
str(out)
# 'data.frame': 4 obs. of  2 variables:
#  $ dressId: int  6 9 10 12
#  $ color  :List of 4
#   ..$ 0: chr "yellow"
#   ..$ 1: chr "red"
#   ..$ 2: chr  "green" "purple" "yellow"
#   ..$ 3: chr  "purple" "red"
out$color
# $`0`
# [1] "yellow"
# 
# $`1`
# [1] "red"
# 
# $`2`
# [1] "green"  "purple" "yellow"
# 
# $`3`
# [1] "purple" "red" 

---

Note: This works even if the "color" variable is a factor, as in Ben's sample data (I missed that point when I posted the answer above) but you need to use I as the aggregation function instead of c:

out <- aggregate(color ~ dressId, d, I)
str(out)
# 'data.frame': 4 obs. of  2 variables:
#  $ dressId: num  6 9 10 12
#  $ color  :List of 4
#   ..$ 0: Factor w/ 6 levels "red","orange",..: 3
#   ..$ 1: Factor w/ 6 levels "red","orange",..: 1
#   ..$ 2: Factor w/ 6 levels "red","orange",..: 4 6 3
#   ..$ 3: Factor w/ 6 levels "red","orange",..: 6 1
out$color
# $`0`
# [1] yellow
# Levels: red orange yellow green blue purple
# 
# $`1`
# [1] red
# Levels: red orange yellow green blue purple
# 
# $`2`
# [1] green  purple yellow
# Levels: red orange yellow green blue purple
# 
# $`3`
# [1] purple red   
# Levels: red orange yellow green blue purple

Strangely, however, the default display shows the integer values:

out
#   dressId   color
# 1       6       3
# 2       9       1
# 3      10 4, 6, 3
# 4      12    6, 1

Assuming your data frame is saved in a variable called df, then you can use simply group_by and summarize with list function of dplyr package like this

library('dplyr')

df %>%
  group_by(dressId) %>%
  summarize(colors = list(color))

Applied to your example:

df <- tribble(
  ~dressId, ~color,
         6, 'yellow',
         9, 'red',
        10, 'green',
        10, 'purple',
        10, 'yellow',
        12, 'purple',
        12, 'red'
)

df %>%
  group_by(dressId) %>%
  summarize(colors = list(color))

# dressId                colors
#       6                yellow
#       9                   red
#      10 green, purple, yellow
#      12           purple, red