3- or 6-digit hex.
Optional leading #, then either 3 or 6 hex digits; #RGB expands by duplicating each nibble to #RRGGBB. Output is a decimal rgb(r,g,b) triplet in 0–255.
This calculator uses plain decimal numbers. If a result looks wrong, check whether you used a comma instead of a dot (or vice versa) where the field expects one.