geom_abline doesn't recognize scale_color_manual colors

I have disparate datasets I'm bringing together with ggplot. I can get the colors for lines and legend to work with geom_line, but not geom_abline. For example, this works:

df1 <- data.frame(year=c(1990, 1991, 1992, 1993, 1994),
                  varA=c(10.1, 12.2, 9.7, 11.2, 10.5))

df2 <- data.frame(year=c(1990, 1991, 1992, 1993, 1994),
                  varB=c(900, 780, 889, 910, 820))


cols <- c("varA" = "#D55E00", "varB" = "#0072B2", "varC" = "#D55E00")

g1 <- ggplot() +
  geom_line(data=df1,
            aes(x=year, y=varA, color="varA"), 
            linewidth=1) +
  geom_line(data=df2,
            aes(x=year, y=varB/100, color="varB"), 
            linewidth=1) +
  scale_color_manual(name=NULL,
                     values=cols,
                     labels=c("Variable A", "Variable B")
  )

g1

But when I try to add geom_abline, it doesn't recognize the named color:

varC_coef <- coef(lm(varA*0.2 ~ year, data = df1))

g2 <- ggplot() +
  geom_line(data=df1,
            aes(x=year, y=varA, color="varA"), 
            linewidth=1) +
  geom_line(data=df2,
            aes(x=year, y=varB/100, color="varB"), 
            linewidth=1) +
  geom_abline(intercept=varC_coef[1], 
              slope=varC_coef[2], 
              colour="varC") +
  scale_color_manual(name=NULL,
                     values=cols,
                     labels=c("Variable A", "Variable B", "Variable C")
  )

g2

I get this error:

> g2
Error in `geom_abline()`:
! Problem while converting geom to grob.
ℹ Error occurred in the 3rd layer.
Caused by error:
! Unknown colour name: varC
Run `rlang::last_error()` to see where the error occurred.
> 

Any ideas what the problem/solution is?

TIA

Update:

Applying some of the suggestions so far from comments, I've gotten this far. I changed it to not use scale_color_manual at all and added scale_y_continuous so the geom_abline shows up on the graph. Even though I explicitly set "show.legend = TRUE", it's not showing up in the legend. ??

g2 <- ggplot() +
  geom_line(data=df1,
            aes(x=year, y=varA, color="#D55E00"), 
            linewidth=1) +
  geom_line(data=df2,
            aes(x=year, y=varB/100, color="#0072B2"), 
            linewidth=1) +
  geom_abline(intercept=varC_coef[1], 
              slope=varC_coef[2], 
              colour="black",
              show.legend=TRUE) +
  # scale_color_manual(name=NULL,
  #                    values=cols,
  #                    labels=c("Variable A", "Variable B", "Variable C")
  # ) +
  scale_y_continuous(limits = c(0, 13))

g2

It appears that geom_abline isn't the best choice for this particular application. To summarize the very helpful comments, geom_abline neither inherits aesthetics from the plot default nor re-scales the y-axis to include itself if it falls outside the range defined by the earlier geom_lines. Even though no aesthetics were set in the top-level ggplot() line, the inheritance comes from the scale_color_manual statement, which uses the named vector "cols". Geom_abline does not participate in those settings.

In order to get the outcome I needed, I switched geom_abline to geom_smooth. This does inherit aesthetics, contributes to the y-axis range, and allows seamless inclusion into the legend. This code works:

cols <- c("varA" = "#D55E00", "varB" = "#0072B2", "varC" = "#882255")

g3 <- ggplot() +
  geom_line(data=df1,
            aes(x=year, y=varA, color="varA"), 
            linewidth=1) +
  geom_line(data=df2,
            aes(x=year, y=varB/100, color="varB"), 
            linewidth=1) +
  geom_smooth(data=df1,
              aes(color="varC",
                  x=year,
                  y=varA*0.2),
              se=FALSE,
              method=lm) +
  scale_color_manual(name=NULL,
                     values=cols,
                     labels=c("Variable A", "Variable B","Variable C")
  )

g3