What about this command:
convert -background black -size 800x480 -fill "#ff0080" -pointsize 72 -gravity center label:'Ask Ubuntu' output.png
By using above command you create an empty .png image which its background color is black.
-background color
set the background color.
the default background color (if none is specified or found in the image) is white.
The created image size will be 800x480(width[xheight]
).
-size width[xheight][+offset]
set the width and height of the raw image.
-fill color
color to use when filling a graphic primitive.
This option accepts a color name, a hex color, or a numerical RGB, RGBA, HSL, HSLA, CMYK, or CMYKA specification. See Color Names for a description of how to properly specify the color argument.
For example,
-fill blue
-fill "#ff0080"
-fill "rgb(255,0,128)"
Enclose the color specification in quotation marks to prevent the "#" or the parentheses from being interpreted by your shell.
To print a complete list of color names, use the -list color option.
-pointsize size
pointsize of the PostScript, OPTION1, or TrueType font.
-gravity type
sets the current gravity suggestion for various other settings and options.
Choices include: NorthWest, North, NorthEast, West, Center, East, SouthWest, South, SouthEast. Use -list gravity to get a complete list of -gravity settings available in your ImageMagick installation.
label:'your Text Here'
write your text into image.png.
Also see more examples.
See full command-line options