Generally, you will notice that compounds containing chromophore and auxochrome groups have a colour. Chromophore groups are groups that are responsible for colour of the molecule once they are “attached” to it: benzene is colourless, but nitrobenzene is pale yellow. Apart from a chromophore group there are groups called auxochrome groups which alter this colour and so the intensity of absorption ( example: nitrobenzene is pale yellow, but p-nitroaniline is a bright yellow to red, the auxochrome enhanced the colour ). Among the chromophore structures, there are groups such as nitro, azo, nitrile, thioketone, conjugated double bonds ( like in retinal, carotene ). Some auxochrome structures are: the aldehyde group, the methyl mercaptan group, hidroxyl group, amine group. To sum it up, functional groups such as these will account for the colour.
Now, onto analysing DNP. This compound has 2 nitro groups and a hydrazo group ( 2,4-dinitrophenylhydrazine ). From the start, we have a colour: red ( if it is pure ). When it reacts with an aldehyde or a ketone in a condensation reaction, a double bond forms between the carbon on which oxygen was attached and the unsubstituted nitrogen of DNP. The formation of this double bond will alter the colour, shifting it to an orange. It also depends on the carbonylic compound you use. If the carbonylic compound contained conjugated double bonds ( like propenaldehyde ), the condensation product will contain conjugated double bonds which are chromophoric ( so in this case I can only guess what the resulting colour would be because we have more chromophoric groups ). So, DNP is red in its purest form. Its colour is the result of the existing functional groups. A reaction with an aldehyde or ketones will shift its colour to orange.