Hypselodoris bilineata has several colour variants. The typical colour form is recognisable according to following characteristics :
- the body is black or dark blue with yellow lines and blue spots
- the mantle pattern consists in 2 median yellow lines encircling the rhinophores and gills at the end. The mantle border is yellow preceded by a black line. The region between the yellow border and median yellow lines is covered in blue spots.
- there are longitudinal yellow lines on each side of the foot
- the rhinophores are black or dark blue with a longitudinal yellow line on the posterior side
- the gills are black or dark blue with 2 triangular yellow lines on the external side of each gill and one yellow line on the internal side of each gill

In northern specimens of this species the median yellow lines tend to merge into a single line.
On the contrary, in African specimens the median yellow lines are separated by a broad region with yellowish white lines.

In Mediterranean and Atlantic Ocean there are 6 similar blue dorids species. Here is a comparative table of characteristics and a comparative draw of the rhinophores and gills.

The northern variant of H. bilineata could be distinguished from Hypselodoris tricolor by :
- its flanks : in H. bilineata they consist in a pattern of irregular yellow lines, while in H. tricolor they have only a whitish yellow line
- its rhinophores : in H. bilineata they have a yellow line, while in H. tricolor they are uniform blue
- its gills : in H. bilineata they have 2 triangular yellow lines on each gill, while in H. tricolor they have only one yellowish line on each gill

The northern variant of H. bilineata could be distinguished from Hypselodoris cantabrica by :
- its dorsum : in H. bilineata it has one or several lines only, while in H. cantabrica it has yellow spots in addition to a median yellow line
- its flanks : in H. bilineata they consist in a pattern of irregular yellow lines, while in H. cantabrica they consist in numerous yellow spots

The African variant of H. bilineata could be distinguished from Hypselodoris villafranca by :
- its gills : in H. bilineata they have 2 triangular yellow lines on each gill, while in H. villafranca they have only one white line on each gill

The northern variant of H. bilineata could be distinguished from Hypselodoris fontandraui by :
- its rhinophores : in H. bilineata they have a yellow line, while in H. fontandraui they are uniform blue

The northern variant of H. bilineata could be distinguished from Hypselodoris orsinii by :
- its rhinophores : in H. bilineata they have a yellow line, while in H. orsinii they are uniform blue
- its gills : in H. bilineata they have 2 triangular yellow lines on each gill, while in H. orsinii they are uniform blue

African Atlantic