Error in plot.xy(xy, type, ...) : invalid color name 'A'

Question

I am working on a paper writing exercise and was provided the following R code to use analyze a given data set. It has worked perfectly until I reach this last step and then I receive this error and I'm not sure how/where I need to redefine the color as I need the scatter plot to be two different colors for the groups. Its the very last line that doesnt produce the needed graph. Thanks for the help.

dat<-read.table("F:/University/NSU_TEACHING/NEW_CORES/NEW_MARINE 
GEOLOGY/NEW_MODULES_2015_7/Online_data/SANDS2.txt", header=F)

par(mfrow=c(1,2))
plot(dat$V2,col="blue",main="Median grain size" )
plot(dat$V3,col="red",main="Sorting Coefficient")
boxplot(dat$V2~dat$V1,main="Median grain size" )
boxplot(dat$V3~dat$V1,main="Sorting Coefficient")

hist(dat$V2,main="Median grain size")
hist(dat$V3,main="Sorting Coefficient")
hist(dat$V2,main="Median grain size",breaks=20)

qqnorm(dat$V2,main="Median Grain Size"); qqline(dat$V2)
qqnorm(dat$V3,main="Sorting Coefficient"); qqline(dat$V3)

shapiro.test(dat$V2)
shapiro.test(dat$V3)

ks.test(dat$V2[dat$V1=="A"],dat$V2[dat$V1=="B"])
hist(dat$V2[dat$V1=="A"])

hist(dat$V2[dat$V1=="B"])
fligner.test(dat$V2~dat$V1)

t.test(dat$V2~dat$V1)
boxplot(dat$V2~dat$V1,notch=T,main="Median grain size" )

plot(dat[,c(2,3)],col=dat[,1])

Data set if needed

#Group A = Beach sands, Group B = Offshore sands, Column 2 = Median grain size in mm., Column 3 = Sorting coefficient

A   0.333   1.08
A   0.340   1.08
A   0.338   1.09
A   0.333   1.10
A   0.323   1.13
A   0.327   1.12
A   0.329   1.13
A   0.331   1.13
A   0.336   1.12
A   0.333   1.14
A   0.341   1.14
A   0.328   1.15
A   0.336   1.15
A   0.327   1.16
A   0.329   1.16
A   0.330   1.16
A   0.323   1.17
A   0.328   1.17
A   0.332   1.17
A   0.331   1.18
A   0.326   1.18
A   0.333   1.18
A   0.330   1.19
A   0.336   1.19
A   0.327   1.20
A   0.324   1.21
A   0.332   1.21
A   0.322   1.22
A   0.329   1.22
A   0.325   1.24
A   0.328   1.26
A   0.322   1.27
A   0.318   1.22
A   0.330   1.17
B   0.339   1.12
B   0.346   1.12
B   0.350   1.12
B   0.352   1.13
B   0.341   1.15
B   0.347   1.15
B   0.337   1.16
B   0.343   1.16
B   0.340   1.17
B   0.346   1.17
B   0.349   1.17
B   0.339   1.18
B   0.342   1.18
B   0.346   1.18
B   0.351   1.18
B   0.340   1.19
B   0.344   1.19
B   0.333   1.20
B   0.337   1.20
B   0.339   1.20
B   0.342   1.20
B   0.339   1.21
B   0.340   1.21
B   0.341   1.21
B   0.335   1.22
B   0.337   1.22
B   0.340   1.22
B   0.343   1.22
B   0.334   1.22
B   0.348   1.22
B   0.337   1.22
B   0.342   1.23
B   0.334   1.24
B   0.340   1.24
B   0.342   1.24
B   0.331   1.25
B   0.336   1.25
B   0.341   1.25
B   0.334   1.26
B   0.337   1.27
B   0.339   1.27
B   0.330   1.28
B   0.334   1.28
B   0.332   1.29
B   0.330   1.31
B   0.334   1.31
B   0.340   1.21

Answer 1

From comments:

I suspect this is a stringsAsFactors=FALSE problem. If the guide was written for R 3.0 and you're using R 4.0 or higher, the encoding of that column will be different. Try adding stringsAsFactors = TRUE to your original read.table call, or change the last line to plot(dat[,c(2,3)],col=factor(dat[,1]))