Using the hud_color for the hud and cl_cross_color for the crosshair duh.

Open the console (°) and write the following examples according to the color you want to use.
For example, using these commands we can get a blue hud and a green crosshair

Here is a list of colors you can use, Enjoy!

Snow – “255 250 250”
Moccasin – “255 228 181”
Cornsilk – “255 248 220”
Ivory – “255 255 240”
LemonChiffon – “255 250 205”
Seashell Seashell – “255 245 238”
Honeydew – “240 255 240”
MintCream – “245 255 250”
Azure – “240 255 255”
AliceBlue – “240 248 255”
MistyRose – “255 228 225”
White – “255 255 255”
Black – “0 0 0”
DarkSlateGray – “47 79 79”
LightSlateGray – “119 136 153”
Grey – “190 190 190”
LightGray – “211 211 211”
Midnight Blue – “25 25 112”
RoyalBlue – “65 105 225”
Blue – “0 0 255”
PaleTurquoise – “175 238 238”
DarkTurquoise – “0 206 209”
MediumTurquoise – “72 209 204”
Turquoise – “64 224 208”
DarkGreen – “0 100 0”
PaleGreen – “152 251 152”
Green – “0 255 0”
GreenYellow – “173 255 47”
YellowGreen – “154 205 50”
PaleGoldenrod – “238 232 170”
LtGoldenrodYello – “250 250 210”
Yellow – “255 255 0”
Gold – “255 215 0”
Goldenrod – “184 134 11”
RosyBrown – “188 143 143”
IndianRed – “205 92 92”
SaddleBrown – “139 69 19”
Sienna – “160 82 45”
Beige – “245 245 220”
SandyBrown – “244 164 96”
Brown – “165 42 42”
Orange – “255 165 0”
DarkOrange – “255 140 0”
Tomato – “255 99 71”
OrangeRed – “255 69 0”
Red – “255 0 0”
HotPink – “255 105 180”
DeepPink – “255 20 147”
Pink – “255 192 203”
LightPink – “255 182 193”
PaleVioletRed – “219 112 147”
VioletRed – “208 32 144”
Magenta – “255 0 255”
Violet – “238 130 238”
Purple – “160 32 240”
Cyan – “0 255 255”
DarkRed – “139 0 0”
DarkBlue – “0 0 139”
Teal – “0 128 128”
Olive – “128 128 0”
Navy – “0 0 128”
Maroon - “128 0 0”