r - ggplot2: How to scale colours on two dimensions, categorical and continuous -

i have data

set.seed(28100)  my_data <- data.frame(cat = factor(sample(c('cat1','cat2'), 50, replace = t)),                       x = sample(1:100, 50, replace = t),                       y = sample(1:100, 50, replace = t),                       value = sample(1:100), 50, replace = t) 

and want color observations based on 2 dimensions. first, want distinguish between categories colouring based on factor variable cat. second, want scale colour based on continuous variable value.

this doesn't give expected result:

require(ggplot2) ggplot(my_data, aes(x=x, y=y, colour=cat, fill=value)) +   geom_point(size=5) +   theme_bw() 

is possible tweaking arguments of ggplot() or geom_point()?