You are not coloring a space here, but characters that you are plugging into a space, but I guess you know that.
Try the following where the longer space replace formula is nested within the shorter one:
replace(replace({FIELD}," ",
"<font color=red>"+"<b>"+"@@@"+"</b>"+"</font color>")," ", "<font color=blue>"+"<b>"+"@@"+"</b>"+"</font color>")
-LB