title: how to change the color in geom_point or lines in ggplot
body: <p>I have a data like this</p>
  
  <pre><code>data&lt;- structure(list(sample = structure(c(1L, 1L, 1L, 1L, 1L, 1L, 1L, 
  1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L), .Label = c("A", 
  "B"), class = "factor"), y = c(0.99999652, 0.99626012, 0.94070452, 
  0.37332406, 0.57810894, 0.37673758, 0.22784684, 0.35358141, 0.21253558, 
  0.17715703, 0.99999652, 0.86403956, 0.64054516, 0.18448824, 0.40362691, 
  0.10791682, 0.06985696, 0.07384465, 0.0433271, 0.02875159), time = c(100L, 
  150L, 170L, 180L, 190L, 220L, 260L, 270L, 300L, 375L, 100L, 150L, 
  170L, 180L, 190L, 220L, 260L, 270L, 300L, 375L), x = c(0.9999965, 
  0.9981008, 0.9940164, 1.0842966, 0.9412978, 1.0627907, 0.9135079, 
  1.1982235, 0.9194105, 0.9361713, 0.9999965, 1.0494051, 0.9526752, 
  1.1594711, 0.9827104, 1.0223711, 1.1419197, 1.0328598, 0.6015229, 
  0.3745817)), .Names = c("sample", "y", "time", "x"), class = "data.frame", row.names = c(NA, 
  -20L))
  </code></pre>
  
  <p>I am interested in plotting it with a costumed color like black and red</p>
  
  <p>I can plot it with two random different color like this but the problem is that </p>
  
  <pre><code>ggplot() + 
  geom_point(data = data, aes(x = time, y = y, color = sample),size=4)
  </code></pre>
  
  <p>if I want to assign the first one (A) to black and the (B) to red. how can I do that? </p>