All these define the color as red

h1 { color:#f00; color:#ff0000; color:#FF0000; }
p  {color:rgb(255,0,0); color:rgb(100%,0%,0%); color:red }

These are the same colour:

h1 { color: #f7a; color: #ff77aa }

• Replicating the character evens the rgb value between 0 and 255

Using decimal percentage values (37.432%) does not work in some browsers

• Some nearly red (99.25%) text works in IE6, Firefox, Chrome, Safari and Opera
• Negative values should be replaced by zero or something sensible
• See www.w3schools.com/html/html_colornames.asp for some web information about colour
• CSS2.1 states that the RGB values are in the sRGB colour space so, in theory, they should be the same on all devices that are correctly set up!
• CSS3 is looking at Colour Management issues. Likely to inherit the colour support in SVG