I made a list of parts to replace to decolor the 10327 Dune Ornithopter (explanation and text version in description)

I made this list based on the instructions PDF that's available from LEGO's website. For most of these, it doesn't really matter if they are black, dark bluish gray or light bluish gray so I picked them based on what will most likely look good with the surrounding colors and what is cheapest on Bricklink.

Two notable parts: Technic, Axle and Pin Connector Perpendicular Double with Axle Connector 2L is only available in white (the same color as in the set) and for the Technic, Axle 1L with Pin without Friction Ridges "old" light gray is significantly cheaper than any of the newer colors except the included tan.

In general, if you want to save a few bucks, you can probably leave all tan and white pieces as they are. For me, the complete replacement (250 pieces) cost about 35€ plus shipping.

CountNumberNameOriginalReplacement(s)
26346389Technic, Axle and Pin Connector Perpendicular Double Split, Reinforced SidesWhiteDark Bluish Gray
56058177Brick, Modified 1 x 2 with Studs on 1 SideWhite4x Dark Bluish Gray, 1x Black
66289797Technic, Axle 2L with Pin without Friction RidgesWhiteLight Bluish Gray
44208160Technic, Liftarm Thick 1 x 3WhiteBlack
26370532Technic, Axle and Pin Connector Perpendicular Double with Axle Connector 2LWhiteNot available
10371001Plate 1 x 4White6x Dark Bluish Gray, 4x Light Bluish Gray
26360069Technic, Liftarm Thin 1 x 5 - Axle HolesWhiteLight Bluish Gray
2389401Technic, Brick 1 x 6 with HolesWhiteBlack
26001831Technic, Liftarm Thick 1 x 9WhiteDark Bluish Gray
46013938Technic, Pin 3/4TanDark Bluish Gray
64666579Technic, Axle 1L with Pin without Friction RidgesTanLight Gray
16117975Bracket 1 x 2 - 2 x 2TanDark Bluish Gray
84121921Plate 1 x 3Tan4x Dark Bluish Gray, 2x Black, 2x Light Bluish Gray
44118790Plate 2 x 3TanDark Bluish Gray
44201062Brick, Modified 1 x 4 with Studs on SideTan2x Dark Bluish Gray, 2x Black
54550323Arch 1 x 4TanBlack
24550324Tile 1 x 4TanBlack
16186258Slope 45 2 x 4TanDark Bluish Gray
24143409Plate 1 x 10TanDark Bluish Gray
76271820Technic BushRedLight Bluish Gray
306378120Technic, Pin 1/2 with Friction RidgesRedDark Bluish Gray
206360104Technic, Pin with Friction Ridges and Tow Ball with Round Pin HoleRedBlack
26384112Bar 1L with 1 x 1 Round Plate with Hollow StudRedBlack
314142865Technic, Axle 2L NotchedRedBlack
7300421Brick 1 x 2Red4x Dark Bluish Gray, 3x Black
16385920Technic, Brick Modified 1 x 2 with Hole and 1 x 2 PlateRedLight Bluish Gray
26271827Technic, Liftarm, Modified Bent Thick L-Shape 2 x 4RedBlack
36130002Technic, Axle 6LRedBlack
16335578Tile 2 x 6RedBlack
2379521Plate 2 x 6RedDark Bluish Gray
16130005Technic, Axle 10LRedBlack
74206482Technic, Axle 1L with Pin with Friction RidgesBlueBlack
206299413Technic, Pin 3L with Friction RidgesBlueBlack
46271167Technic Bush 1/2 SmoothYellowLight Bluish Gray
14654124Technic, Brick 1 x 1 with HoleYellow8x Black, 6x Dark Bluish Gray
16344181Plate, Modified 1 x 2 with Pin Hole on TopYellowLight Bluish Gray
86130007Technic, Axle 3LYellowBlack
16072627Technic, Liftarm, Modified T-Shape Thick 3 x 3YellowLight Bluish Gray
2362224Brick 1 x 3YellowBlack
26170294Technic, Liftarm Thin 1 x 6YellowBlack
46474196Bar 1L with Tow BallOrange?Dark Bluish Gray
44164022Brick 1 x 2Lime Green?Dark Bluish Gray
14164037Plate 1 x 2Lime Green?Dark Bluish Gray
54537937Plate 2 x 2Lime Green?1x Light Bluish Gray, 4x Dark Bluish Gray